Bullies exploit an unfortunate game theoretic equilibrium.

You'd rather the bully not notice you, but if he's going to notice you you'd prefer for him to think you're on his side.

That's the incentive for each individual.

But that means that everyone puts their head down and lets the bully get away with whatever he wants.

The more that everyone puts their head down, the stronger the disincentive for everyone else to stick their neck out, because they'd stand out prominently and definitely get smacked down.

In some contexts there's a bigger authority that can smack the bully down.

But if there isn't one, the only way is for everyone to rise up against the bully at once.

That can happen if there's a supercritical state where everyone is upset but no one wants to stick their neck out.

Those supercritical moments are prone to catastrophic shattering when the right inciting incident comes along that everyone simultaneously can see everyone agrees the bully has crossed the line.