"Always assume other drivers are going to do the WORST possible thing"
I often get tailgated in the HOV (in cage) because I won't haul a** past the slower moving traffic in regular lanes, just for this reason. I go faster than them, but not hauling (keeping enough stopping distance). You just never know when someone's going to do this.
Heck, I'll cross into the HOV lane (after a very careful check) - I have no problem with it (though our lanes are white lined, not yellow).
In the vid I didn't see him do ANY braking - his hands never moved. Given what was going on in front of him he should've at least been slowing down/off the throttle.