I suspect something definately let loose on one or more of the links. You can see the axle locks up before the rig rolls over it, so the brakes were working, and you hear the motor rev right when he starts to roll backwards, leading me to believe either the rear driveshaft snapped as a result of a broken link, or leading up to it. It's tough to tell whether or not the front wheels were still digging, but the brake lights came on as soon as it started rolling back down the hill too....
My theory is that the link broke, the axle rotated, snapping the driveshaft, which was when the engine revved and it started rolling backwards, then he switched pedals, stood on the brakes which stopped the axle, but because it was no longer linked to the truck, the truck kept going... right over top of it.
Still funny stuff....