The theory is that bullies are often in the middle of a bullying hierarchy. For example, when I was in high school, one of my friends was harassed by seniors when he was a freshman. When he became a senior himself, he, in turn, harassed freshmen, saying that he was going to give as good as he got.

From what I've read, in high school at least, bullies tend to be those in the middle of the social hierarchy; those at the top (the most popular) are secure in their position and can afford to be nice, while those who are at risk for backsliding work hard at making sure there is at least one person who is a more tempting victim than they are.