You ever get stuck in traffic? You step on the brake, right? What would happen if you didn't? You'd tap the guy in front of you, who might hit the brake, or might hit the gas and hit someone else, or might hit the brake and still hit someone else. That's how you get a 4 car pileup in dead stop traffic.

When you're in a human crowd, though, you can bump and push people in all directions. And they'll move, too. And all of a sudden, people are just pushing forwards to escape the pushing behind them