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?
Examples:
Deck of cards:
https://www.youtube.com/watch?v=ETjVjhZOLSI
Paper sheet folded repeatedly:
https://m.youtube.com/watch?v=KuG_CeEZV6w
A book:
https://m.youtube.com/watch?v=PmvKlnhMjUw
Another book, notice the heavy metal object in the background being launched into the air:
https://m.youtube.com/watch?v=RoVxbLLQdPU
Best Answer
Assuming the deck doesn't bulge to much at at the sides (as the video suggests), the press exerts work $W$ on the deck acc.:
$$W=\int_0^yFdy$$
With $F$ the force exerted by the press and $y$ the displacement.
This work is converted to potential energy and stored in the deck.
As suggested in the answer to this question, the high pressure is likely to destroy the structure of the paper, reducing its mechanical strength.
An important factor may also be that new, good quality playing cards are very slippery, so that pieces of card can then convert some of that potential energy into translational (horizontal) kinetic energy. (Note that when using ordinary paper, no such 'explosion' takes place)
This may also explain why after the 'explosion' a considerable amount of card material is still found between anvil and press: the sudden departure of material also reduces pressure very quickly and the process of 'explosion' then stops.