It's a complex phenomenon that the psychiatric community is just finally beginning to look at. Bullying is a stalking behavior and so "participatory bullying" is not too different from "participatory stalking" which is more commonly referred to as gang stalking.
Why would people participate? Some for fun. Some because they see an advantage. In terms of school bullying it possibly makes them more "acceptable" to the bully who often, strangely, is the "big man on campus." Or big woman.
The point is there is always one person behind it. Who sets it in motion. With regard to stalking, it is part of the retaliatory response to the victim having cut them off or having filed a complaint. With regard to bullying, I'm not sure anyone can really answer that except that the bully draws a sense of power by bullying someone who is perceived as different from everyone else.
Stalking is about control. And power. And so most likely most stalkers were also bullies. But sometimes they were the person bullied.
The problem is everyone focuses on the victim and the effect on the victim. Not on the victimizer and the motivation for the victimizer to become a predator. And both the bully and the stalker are predators. And we are their prey.