Because they're in a crowd, a really densely packed crowd. They might see that the crowd up ahead of them has slowed down but the people ten, twenty or a thousand meters behind them haven't. So the guys behind them keep pushing forward, and if you try to stop you'll be knocked down and walked over yourself. That is if the pressure of the mass of bodies doesn't crush you to death.
Add panic to that, even if only a few people do, and you've got a recipe for disaster.