My friend's study table looks like this. Why does this happen? Why does the wood under the paint swell and the paint peel?
Although it is difficult to tell by the picture, the damage shown is typical of what happens when painted particle board (which swells when exposed to moisture) is allowed to get wet.
Why does it expand? Because it is made by compressing sawdust with a binding resin, and unless it is protected from moisture the binder is compromised and it "decompresses".