I know it's not getting down to the reasons for it, but I remember something Bathsheba said once (maybe more than once!) about kids being mean or lashing out at other kids.
She suggested that she 'tends to the injured' and effectively ignores the perpetrator.
I can't remember if she said to deal with the perpetrator later, I guess from there you need to look at approaches for bringing it up and talking about it that will work for that individual child. The crux, however, was that the perpetrator sees that by lashing out, they effect the end of someone else getting all that attention and experiencing a level of ostracism because of their actions. It's a kind of natural justice, I tend to think. Left on their own at a slightly older age (say about 5 and up) kids will ostracise such a child anyway and play amongst themselves anyway.
I don't know how much this helps, but I"m starting to do this with DS, as he's starting to throw his weight around from time to time.