Sometimes, that “deer in the headlights” expression is somewhat invalid, especially when you’re dealing with a surprisingly brave buck.
In this instance, the animal ran straight into the road without freezing in front of those riders. Unfortunately for one of those guys, he lost control of his motorcycle just as the buck leaped from the road on to the grass.
In a way, the title of the clip is a bit erroneous. The animal did influence what happened to the rider, however as far as we can tell, it didn’t exactly “take him out” directly.
What happened is that the rider climbed on the brakes before the buck actually got near him, followed by the bike starting to skid just as the animal was executing what can only be described as a very impressive long-distance jump.