I would jump to the conclusion it was the fault of .... the driver who took off.
Just because they left the scene doesn’t mean the accident was entirely their fault. They could have been illegal, have warrants, were DUI, etc., that’s why they left.
I would think they were at fault because of the speed and angle that they came in, possibly a hard turn from the HOV lane at high speed and Nick1994 didn’t see them in the mirror because of that maneuver. But I’d bet a cop would be hesitant to lay blame solely on one or the other if the other vehicle had stayed at the scene.