Well, a lot of the bullies have insecurities and this leads to them projecting their self-hatred onto others as a form of coping mechanism to battle their own. They pick on people they find weaker because they want to gain a sense of power for all the times they felt powerless about the things that happened to them. But, many a time, when the victim retaliates, they may often get scared, either due to the unexpectedness of the situation, or their feelings of insecurity/powerlessness.
*(depends on their situation/the way they grew up)
*(reply based on an article I read long ago)
*(if you already knew this and meant it as a joke, welp soz~)
northstar