Brake Checking itself is illegal basically everywhere because it would constitute reckless or dangerous actions that caused or could have potentially caused an accident.
However, whenever you hit something from behind. You are automatically assumed to be at fault unless there were other mitigating factors; because we are never supposed to be driving so fast that we can't stop when something crosses our path or stops in front of us. That's not realistic, but it's the law.
But...in this case. It's pretty obvious the driver of the motorcycle initiated the Slowdown intentionally and not to avoid anything on the road. So, he has some culpability.
I would guess a judge would also probably have some amount of culpability for the driver of the vehicle because their reaction to the reckless driving wasn't to slow down and deescalate the situation..but to maintain course and speed.
There are mitigating circumstances but pretty obvious the car driver didn't do everything possible to avoid an accident.