Ripple effects. It just takes a little bit of panic in one spot and people try to leave +trying to make more space and than it spreads out in every direction like throwing a stone in water. That leads to other points of too little space and therefor panic. So it just spirals out of control. There is also confusion and panic why some people trying to get away.

If everyone was rational, everyone would take their arms down. Take as little space as possible and try not to move anywhere. and it would be over in 10 sec.