Of course you heard about the Wal-Mart worker who was trampled by shoppers.

I've been in one of these crowd situations. I (and about five hundred other people) were waiting for the last commuter train out of town one cold, rainy night. The last two trains over the prior hour had been canceled because of mechanical problems, so nobody was in a good mood. The door dividing the waiting area from the track area was about seven or eight feet wide, secured by a locked, sliding door. When the train was announced, everybody surged toward the door. I was near the front of the crowd. When the door opened, everybody started moving toward the bottleneck.

Once it started, there was no way to stop. The people behind pressed forward, and the people in front had to keep moving. If somebody had fallen (luckily, nobody did) there would have been no way to get him back up because of the pressure of the crowd pushing from behind. It wasn't that there was any malice involved, it was just a matter of crowd dynamics and physics. It was as if each person pressed on the one in front with an pound of force, but the cumulative effect was a ton at the front of the crowd.

http://www.crowddynamics.com/Main/Crowddisasters.html

http://www.wikihow.com/Evade-a-Stampede-of-Shoppers