Great piece by Emily Badger about the physics of crowds and why fatal stampedes happen, even in crowds comprised of peaceful, reasonable people:
The small movements of so many people aggregate into a powerful force – one that security officials are often helpless to halt – that has the capacity to knock over bodies, shove them together and, ultimately, asphyxiate them. This sounds impossible, but 21 people died at Love Parade inside a crowd that had essentially been standing still. There was no real crowd rush or dramatic “stampede.” And this is the heart of the mystery to non-scientists as to how such a thing could happen.