Alright, your picture made me understand what you're talking about.
Its the moisture that's doing this. Paper is hygroscopic--it absorbs water from the atmosphere, if only a little bit.
Now, paper is wood (pulp). And wood contains plant cells. Plant cells(or whatever's left of 'em) absorb this water and swell. This causes the paper to "warp" (you may have noticed this while using watercolors--or just dunk a scrap of paper in water and see what happens when it dries). This warping is due to the fact that the paper has limited area, so the molecules have nowhere to expand but up/down.
Now, if you have a bunch of sheets of paper, the warping will not be the same for each one--as in the warped "humps" will not necessarily fit into the warped "troughs" and vice versa. Since the sheets no longer have a snug fit, we have a lot of extra space. This causes the book to "puff up". The corners have more freedom than the rest of the page, so they can at least curl away when puffing up.
I once got a book (physics book, incidentally :/ ) wet in the rain when I'd kept it in the non-waterproof section of my bag. It puffed up like crazy. After drying it and flattening it, it's no longer puffed up as much, but the edge of the book is still pretty U-shaped.