I dunno, it seemed pretty obvious what happened. The car reached the end of the chain, no one was there to drive it off, someone got in the car behind and honked the horn (which you can hear), so one of the wash jockeys jumped in the car and must have done the "unintended acceleration" thing, thinking he was stepping on the brake when he was actually on the gas. When the car comes to rest you can just see on the dash one of the green towels the wash jockeys were drying the car with.