Looks to me like he over loosened because the chain was hung, then it started to move and he clamped down on the brake. Trouble was, the brake material started to overheat and when it does it looses coefficient of friction, the result was brake fade and as the brake faded he increased pressure until it burst into flame. At that point all was lost because once the braking material is in flame it will not grip. We in North Georgia see cars, usually with Florida plates loose their brakes on the down grades such as "Tail of the Dragon" as they constantly ride the brakes going down hill. The hotter they get, the more they brake, which makes them hotter, they brake more and then they start honking their horn and they go by with brake discs cherry red. LOL!