I found another very good example (even worse than my stamps) on an auction site. You can clearly see how many of the stamps are bending out in a U-shape or whatever you want to call it. While some lay flat. Im convinced that it has to do with the gum in combination with maybe temperature as many of you already stated. And as I said before when they bend out like this they become stiff and crispy-like.
Another theory is that the hinge is causing this or at least making it worse.
I forgot to ask, why is it that some stamps watermark becomes more visible, and some stamps colors goes through to the back???? Almost like the gum is dissolving, or is thinning out making it more translucent.