There's a certain scale where a group of people turn into a mob. And mobs are ruled by the id, a perfectly normal, conscientious person's behavior transforms when part of an anonymous mob or when interacting with a faceless entity in an impersonal way.

When I worked as a retail salesman, you'd see it all of the time. From people pushing expired coupons to return fraud to outright theft, people degrade in that sort of environment.