Not to discount the ill conceived pass but, if the the turn signal bulb in the car was even working...
...and with no traffic oncoming, why did the car need to come to a nearly complete stop to turn from the highway?
I'd be willing to bet if it had been an 18 wheeler coming up behind the turning car that;
they would have signaled long before the turn and gotten right the fuck out of the way.
Unless they have a death wish or really are that oblivious to their surroundings.
Rider was certainly not paying close enough attention, just the same.
Just so much speculation since none of us has the full picture of what happened leading up to the impact...
...from each pilot's perspective.