This clip is from the World Cup Downhill Race in Cairns. OUCH! I hope this guy is ok, especially considering he was not wearing a helmet! And here’s some details around how this happened from Reddit:

The pro rider, Adam Brayton, crashed further up the track fairly hard and couldn’t finish his run so this bloke volunteered to take his bike back to the pits. Instead of pushing back up the steep, stupidly muddy track he decided to ride it to the bottom (as per usual with this sort of situation at state/national level). Most likely drunk (like most other spectators this far into the day) and encouraged by the crowd, he tried to show off/ride fast and binned the fuck out of it.