No doubt at all it was driver error on many points including speed. I have pulled that exact grade prior to the wreck with multiple towing vehicle's from logging /lumber and grain trucks to a multitude of RV's from motorhomes/fifth wheels/pickup campers and travel trailers... Its home!
I have no trouble maintaining the speed limit over that grade with my whimpy 2008 5.4.His unit was newer.
I have yet to here one reply based on experience towing on that road pulling a trailer. Not one but a lot of guessing not knowing the road or anyone involved. The speed limit is 65 mph and passing a semi at 65 mph on a two lane road is stupid because the speed needed to pass that semi going 65 mph is beyond what a person should drive pulling a trailer on that road.
If you were not there or you do not have first hand knowledge of that accident, you don't have a clue what happened other than the report that does not go into detail.