I'd guess the two cars in shot were impatient to pass, maybe after following the outfit for a while. This seems possible because the silver one pulls out so rapidly just after a car coming against them has barely gone by. Equally likely that the guy in the other 4 wheeler was just as determined to get by the outfit.
Also, could be that the poor outfit guy had these characters up his pipe for a while. Poor bugger is then the total innocent in their bit of crass stupidity. Wonder what the camera vehicle did about it, and why did it have a camera operating ?
Proves that you can do all you can to reduce, as well as avoid serious mishaps, but you are still at the mercy of mindless cretins