This is just a fast spinning wheel from the descent hitting the ground and spinning the (v lightweight) bike around.
Used to do this as a kid messing around - turn the bike upside down, spin the pedals to turn the wheels as fast as possible then push the bike over and watch it spin around :P
Marca suggesting Hesjedal may have a motor in his bike on the basis of the footage below:
http://www.steephill.tv/players/youtube3/?title=Ryder+Hesjedal+crashes+out+of+Stage+7+break+then+cameraman+drives+over+his+bike&dashboard=vuelta-a-espana&id=OBvgUBpJSkk&yr=2014
Seems weird that the bike starts turning again after it appears to have come to a stop.