Taking a look from the bullies perspective, they often do this because of past things people have done to them, also, some of them feel the need to stay on the top, and therefore push others down. This can be caused by phycological stress and previous experiences from being bullied themselves, also often these people set themselves apart from all social groups, making themselves at the top of the hill from their point of view, but to everyone else they're at the bottom, this become more apparent as time progresses.
