I do the crash search all over North America including Canada,and am always very sorry to see any of these reports when a trucker is killed.
Now being a retired UPS safety trainer, I thought he either fell asleep at the wheel,or it could have been medical like you think bashahogue.
The other thought I had was he could have been cut-off,by a speeding car,or another Semi,which can happen so fast he wouldn't of had time to hit the brakes.
God have mercy on you,and the rest of his family,friends,and co-workers,may God grant you peace that whatever may have happened,the authorities will figure it out,and give you some closer.