Okay, so looking into it, the way it works isn't quite as impressive as it looks, but it's still damned impressive. Basically rather than there being rocks already on the hill that start sliding when disturbed, whenever there's a disturbance the hill spawns a bunch of rocks that previously didn't exist, and those are what slide down the hill (I think the first 2-3 rocks were already there, but the rest are spawned as needed). They use the motion blur and the dust effects to cover up the spawning. Still looks amazing though.