Look at something as simple as a gig. Thousands of people all trying to get as close to the front as possible. People have been very seriously injured at the front just by virtue of the rows upon rows of people all shuffling forward behind them. The force of a crowd really is staggering.
^^This^^
I remember at a Foo Fighters gig when we were getting smashed forwards from loads of people pushing from behind and i did feel a bit panicked to the point where i wanted the current song to end purely so i could move away from the front. When you literally cant control where you are going and are moving with the crowd as everyone is moving as one it can be quite disconcerting.
The difference with modern gigs is that people regularly climb out/escape at the front and said barriers and stewards are designed to handle this.