I had figured that the most plausible explanation was that elastic energy was systematically getting stored in the folds, largely because the folds were at right angles to each other (hence each one is a "spine" which forces the sheet to "stretch" to accommodate it). A prediction of this hypothesis is that a book would not catastrophically fail until a much higher pressure, and that it might fail like other objects do, by eventually squirting outward like putty.
It's possible that a hybrid approach is needed: this failure seems like a release of stored energy; this explanation needs to account for the energy storage in the cellular matrix (probably not that complicated) and then we have a ready explanation of why there is a catastrophic failure whenever the hydrogen bonds themselves begin to rip (as all the stored energy is released at once).