The physics of crowds can kill

Almost two years ago, 21 people died when they were crushed to death in the crowd at the Love Parade music festival in Germany. Now, scientists have been able to pinpoint exactly what lead to those deaths. Here's a hint: It wasn't a stampede, there's no evidence of intentional pushing, and it doesn't look like mass hysteria had anything to do with the deaths. So how did those 21 people die? Physics.