the physics of posters?

A week ago, I put two posters (both from the same source) on my wall with thumbtacks. I was very careful to smooth the posters along the wall so that they would lie flat.

Then, a couple days later, I was very dismayed to notice that posters hanging somewhat slack. I even considered moving the thumbtacks in order to stretch and smooth them more so that they would lie flat.

Luckily, laziness prevailed. Today, the posters are lying flat again, just as I hung them.

So, what's the cause here? When the posters hung slack, I assumed that either a) my recollections of hanging them were inaccurate, or b) the posters were heavy enough to widen the tack holes, and thus hang more loosely.

Now that the posters are lying flat again, the natural assumption is that some kind of expansion and contraction occurred. And, the posters are hung in a small room with a small window that is frequently open, so environment inside changes with that outside.

Here's the puzzle, though: In general, temperature change causes bodies to expand w/ increased warmth and contract with cooling. But, if anything, the intermediary period between the first hanging of the posters and today was cooler than either their hanging or today. What to make of this? Perhaps not temperature, but moisture is to blame? How exactly would this cause such contraction and expansion in a wall poster?