My popcorn bag self-vacuumed all the air out of itself. What the

I'm not bothered by this, rather, amazed :P I was popping some corn, normal butter corn, and when it was done, I removed it from the microwave, like always, but, moments after I did so, it almost instantly started crumpling up. Turns out, all the air ( Most of it anyway.) was suddenly sucked out. I don't know how either :P ( Sorry for bad wording, tired.).

Any idea how this could have happened?

I can only guess, but it's probably because the hot air inside cooled and contracted.
Pop some more corn and put it in the refrigerator right away and see it happen again.