Various videos online show paper/card suddenly releasing energy in an explosion like way and transforming into a brittle material when put under sufficient pressure from presses.
Examples are shown where the paper/card is stacked into a pile, folded many times, or is a pile of cards or a book.
It's hard for me to be exact from the footage, it seems that in each case an abrupt change in the material occurs at a certain pressure reducing it's size, perhaps this is a collapse, in some cases pressure is abruptly released outwards and the material even seems charred afterwards. In the case of the playing cards, a lot of the material is shredded into tiny pieces, quite uniformly and at a speed that even in slow motion is perceptibly an instant.
Q: Why does paper/card "explode" and transform when put under stress in this way?
Deck of cards: https://www.youtube.com/watch?v=ETjVjhZOLSI
Paper sheet folded repeatedly: https://m.youtube.com/watch?v=KuG_CeEZV6w
Another book, notice the heavy metal object in the background being launched into the air: https://m.youtube.com/watch?v=RoVxbLLQdPU