It was also on a pretty steep downhill slope, which by itself could have been enough to cause this. You put a wheel on a slope and it's going to try to move do

It was also on a pretty steep downhill slope, which by itself could have been enough to cause this. You put a wheel on a slope and it's going to try to move downhill. It'd be suspicious if the bike had moved uphill, but the video clearly shows it moving downhill.