Because George lost an ear.
Law of authorial consequences. Ha.
No, I don't really have an answer either, but it could be something about (since he was one of the two weasely twins) symbolizing transience and the lasting effect on the mirth of the world that evil (and having to battle it) results in. I'm a writer, but I haven't the foggiest on HER reason for it.
It could just be that life is sometimes very painful and unfair.