The problem is that people don't stop immediately. You stop, the person behind you stops just a little bit later, the person behind them a little bit later than that. As you're walking the gaps in between people shrink. Finally the person in front of you stops, you try to stop but the person behind you is close and bumps in to you a bit as you try to stop, you bump the person ahead of you who has stopped because they can't move forward, which squeezes the person ahead of him. Repeat for thousands of people and you have a collection of tiny small bumps and pushes that squeeze people to the point of death.