User blog:Jmcdavid/Vandalism...Why?

Okay, yesterday we had quite a fiasco. User:A Mystery User was going around deleting content from a bunch of pages and replacing it with stuff like "grrrrrrrr" or "i hate you", which was blatant vandalism.

What I want to know is why someone would do something like that in the first place. It makes absolutely no sense to join a wiki just to be a vandal, and that's literally the only thing he ever did upon joining this wiki.

Does anyone know what could compel someone to do something like that? Because it's a mystery to me, and it drives me nuts not knowing what motivates people to do stuff like that.