Wait until the end

Guy on the right started slowly pushing towards the one on the left until they collided.
Edit:
Found a short article on it:
https://deadspin.com/motorbike-racers-crash-into-each-other-pull-over-star-1833553507

Here you can't see enough, check out thelink shiro posted above. The guy on the right, that stayed on his bike and got punched, was a fucking asshole

Because he pushed away the other guy's bike?

The guy on the right was apparently not paying any attention and started shoving the other guys bike. Then, when he realized what was going on, he continued to push the other guy rather than change his course, figuring the other guy would get out of his way instead. They got caught on each other, the guy on the left got pulled off his bike, and kicked it away to keep it from harming his legs. When the guy on the right finally stopped the guy on the left had had enough and punched him for being an ass.