There's also a good New Yorker article that made a lot of the same points, but in the form of a longform article, if you prefer that: http://www.newyorker.com/magazine/2011/02/07/crush-point. The bottom line is that past a certain "person density", crowds are actually governed by a lot of the same laws as fluid dynamics. When you introduce a "wall" in front of the flow, the people at the back don't even know it because they're too far away to notice. They can't know to "stop" any more than water at the back of a pipe can suddenly stop if the water ahead of it encounters an obstacle.