> Who is selected for bullying is determined not primarily by the bully, but by the social group as a whole.
I disagree. I think that the person who is bullied is primarily selected by the bully, and the only influence that others have is that the bullied person doesn't have enough (or large enough) others around them in order to defend them. Others may then pile in once the target has been selected, but it's not in any way a collective decision.
You could just as well say that society chooses the people who get mugged, or the people whose houses are burglarized, or the people who are raped or murdered. I'm sure you could come up with some neo-Freudian way to convince somebody that makes sense, but it doesn't make sense. It's generalization to the point of uselessness if not complete absurdity.
I was bullied as a child. I was picked because I was an easy, bookish target without many friends, and definitely without tough friends. The bullying ended when I hurt a bully in a way that everyone found out about, and that state was maintained when I made a group of friends who would have defended me if a bully had approached me. The cause of all of this was obvious, not subtle or mysterious.