Do you guys remember how when Harry was younger, like pre-Hogwarts, how sometimes he could make things happen without knowing why (his hair growing back, snake at the zoo, etc.)? Well, I think taht maybe when the two boys were younger, Diddykins was beating on Harry, and Harry might have done something to him (like worked his magic without meaning to). I know that if I were being bullied everyday by my "family" and especially Dudley, I would begin to think some pretty bad thoughts. So maybe one day while Dudley was being Dudley Harry thought one of those thoughts and it magically happened and scared the bejesus out of Dudley. And maybe Harry doesn't remember it because he was so young, but Dudley does because it's so frightening.
Or maybe I'm wrong and we just dont know what it is yet, you know, either way
