They all go running through this narrow valley in a barren desert...thousands upon thousands of faithful muslims....then they pick up rocks and huck them as hard as they can at these big pillars of stone that they pretend is satan.
This year 100 people were trampled to death. The festival happens every year but the mass tramplings only happen every 3 years or so. I guess during the run a few people stumble and hit the ground and from there the trampling just sort of snowballs as more people stumble over the bodies before them. They put their sandles into the bodies and throats of those lying underfoot in their "religious" fever hysteria to throw their stones at their symbolic "satan pillars".
The whole thing is stupid.
Please don't get me wrong. I have nothing especially against Islam or muslims but come on, what the hell does god care if you go running through a desert to huck stones at a pillar of rock you pretend is satan. God is the ultimate creator of the universe in all its glory and complexity. Why would he want us to do something so stupid?