Why is this happening?

Are they on stretched canvas?  I know it's a traditional support, but it's far from being the best one.  It's most unlikely to have anything to do with sunlight, which unfortunately means that the problem is more deep-seated, and will be almost impossible to solve - certainly I wouldn't try to solve it; if I really valued the paintings highly, I'd seek out a conservator. The usual reason for delamination like this has to do with the use of Zinc Oxide, i.e. Zinc White, in the base layers.  The technical explanation is not easy to explain, suffice it to say that Zinc is very bad for oil paintings - especially on a flexible surface; it is an extremely brittle paint, hard and inflexible ("hard" in this case not being the same as durable).   There's a further problem - paint-makers in the past used, and though they should know better some still use, a quantity of Zinc in their Titanium White; it was also added to Flake White (the genuine lead pigment).  There's a lot of research going on into this, to determine if there is any safe level of Zinc in other whites.   Varnishing oil paints MAY help to stablize them - until the time comes to remove the dirty varnish; then, it can pull the paint off.   What if it's not the use of Zinc White, but the other disastrous error*: varnishing before the paint has cured? It is essential, and I don't care what the manufacturers of Gamvar say, NOT to varnish before at the very least 6 months have elapsed, and preferably not before 8 to 12 months.  If the painters of your paintings varnished too soon, especially if they used Zinc as well, they unfortunately sealed the destruction of their work into its very fabric.  There is nothing any amateur restorer can do about this - and not much a professional can do either. Can you photograph the paintings for us - it may just help; and identify the artists if they're at all well-known: it would then be possible to check whether other works of theirs have suffered this way, and what - if anything - was done about them.  Very sorry this has happened to your paintings.   * Error: the use of Zinc would be an error today, but not necessarily so when your paintings were done - while it was always known that Zinc White is a very slow dryer, and it would have been a mistake to use it even if very heavily diluted as an underlayer, the tendency of the paint to produce metallic soaps which destabilize the painting surface has only really become well-known in the last few years.  There may be many paintings out there, especially on flexible supports, that are likely to suffer the same way.  If anyone is worried about this, take some comfort from the fact that it doesn't always happen - Millias's painting "Ophelia" is known to have a significant area of underpainting using Zinc White; it's still in very good condition.

Without closer inspection, the reason for this is difficult to determine. As it’s happening to paintings by more than one artist possibly means that it could be an environmental issue, heat, sunlight, humidity etc. There’s no cheap and easy fix unfortunately, but I’d certainly see what your artist friend has to say. I have done some conservation work in the past, but mainly cuts and so on in the canvas, so a relatively simple fix! Individual flakes of paint can be removed obviously, and repainted, it’s time consuming and a degree of patience is required. Good luck anyway Sandra.

Thank goodness that Alan answered, because I just didn't see your reply and you might have thought me rude for ignoring you.   I'll do my best to supplement Alan's advice: I can't see the problem in the first painting, it's not immediately clear where the paint has flaked away.  On the second painting, I presume those trees in the top left quartile aren't meant to be blue.  Would overpainting work, you ask - well, that would depend on the reason for the delamination of the first layer; I'm not a professional conservator, but - if the price of one of those is beyond you, or you can't find one (I expect NZ universities would have departments which could advise on this: they might even do it at nominal cost, if it advanced their understanding of paint chemistry - got to be worth a try!). You - that is, the artist - could overpaint; sedulously avoiding Zinc White; lead white would be better, if it can be got.  It may be that some abrading of the paint over which painting is to be done would be helpful.  I don't know how long that would last - it may last forever!  And it may not.  Helpful, eh.....? I'm assuming - sorry if you've said all this already, it's a bit difficult going back and forth up the thread: here's where two screens would help - that these are oil paintings from cradle to grave: i.e. that the underpainting was oil, and of course that the added paint was oil, and not acrylic.  If the latter, well of course the whole painting is unstable.  If the former - it has been widely assumed and often stated that oil over acrylic is safe; however, that is by no means certain; only the passage of time could demonstrate that, and acrylics haven't yet had that time.  I have no issue with painting on an acrylic primer (often called, wrongly, "gesso") but acrylic paint is not made the same way as the primer, and there are certainly some acrylics I would never use under oil, because I would expect the more fluid ones, and the more finely ground colours, to cause delamination. Only the artist will know if they used any acrylic colours in the underpainting - and they might well have forgotten, given the age of the paintings.  Of course, a conservator's analysis would reveal that, if you can arrange for it to be done.   On the whole - a painting that was started and finished in oil (and in which no wax was used) can be overpainted/restored.  So getting the artist to do that does suggest one way forward; I'd prefer the conservator route if it were possible, but overpainting is the second best (having the advantage that it would still be all the artist's own work - it would be very interesting though if they could work with a conservation department: both could learn a lot of very useful processes). My suspicions here - especially given these were not flexible surfaces - rest with a) Zinc White, applied in the ground layer; b) an unsuitable acrylic used for the underpainting; c) the almost impossible outlier possibility, that this was a case of acrylic over oil: not a mistake any experienced painter would make, and I'm sure the artist in this case didn't; d) also pretty unlikely, that the upper layer of paint was under-bound or thinned with Turps, over an oil-rich layer.   It's very unlikely that atmospheric conditions have anything to do with it: excess heat, damp, extreme cold, can cause problems - but I've never known them to cause issues such as these.  Why you have two paintings, by different artists, suffering the same problem - that is a real mystery; this is why I'd really love it if you could afford a conservator to look at both of them!  I'm itching to know - you see, I've kept some of my old oils that weren't successful as paintings (i.e. they were rubbish!) and have kept them in very adverse conditions where they've got dirty, mouldy, even damp around the edges, so that I could see what might be likely to cause them serious problems; some of them are around about the same age as yours, and none of them has so much as cracked yet; I'm looking at one now, which I keep in a disused fireplace - it's not looking happy, but I could restore it to its unlovely best if I wanted to, with a good clean and a bit of a touching-up: the paint isn't falling off, though the white I used would have been Titanium, with a modest addition of Zinc.  The damage done to your paintings is unusual, untypical, and it's screaming for expert investigation. I'll add a footnote in another post, because this is already a huge post to wade through.

The footnote; you say you've got a good answer to your question about varnishing, but just to finish the job I've started - how long a coat of varnish lasts depends on the conditions in which the painting has to live; dirt is the usual problem, and if that can't be cleaned off with a cotton swab moistened by - well, the best thing is spit! - it's time to remove the old varnish and add a new one.  Mineral spirits will remove most modern varnishes; the one you can't remove (usually) with OMS is dammar: that needs genuine Turpentine, and is much better left, if you have to remove it, to someone who really knows what they're doing. No varnish should be added before 8 months have elapsed - I know the makers of Gamvar claim that it can be applied when the painting in touch-dry, but while Gamblin is a well-respected and experienced company, I don't agree with the use of their varnish on "young" paintings.  It won't kill the painting, but problems could well arise when the time comes to remove the varnish, which may be 20 years down the line, but you still don't want it happening.  I don't trust Retouch varnish, either;  just for completeness' sake.  Once again, I do apologize for not replying to your renewed questions, someone who dug up an old thread managed to push it down the page and I didn't think to look for it.  Needless to say, I agree with Alan's points (and he wasn't the one who pushed the thread down the page).  

Oh, PPS = (once I start, I'm hard to stop): varnish has two important effects: it enhances the colours, and it protects against dirt and abrasions.  A good modern varnish (or glass) will also protect the painting against UV light, which can fade colours.