Three explanations of collective/crowd behaviour
Le Bon: When in a crowd, people lose their sense of self, responsibility, and morality. Behaviour is unconscious and driven by instinct so people act in ways that they usually wouldn't.
Reicher 1984: Crowds act under a common social identity. They attack those in the "out-group", but not those in the "in-group" (similar interests, culture, etc.)
Reicher 1996: Crowds turn violent when police use heavy-handed methods to control the crowd, causing anger due to the shared in-group experience, so now have a common group goal against the police.