Immediate LOC could result in what we saw, but not likely is what I am saying.
Class 8 trucks are not excatly a glass smooth ride like a car. That along with the outreached driving position would tend to make one fall forwards or to the side, since they are supporting their weight in part by their arms on the wheel or arm rest. Once you lose control of those muscles, only a miracle is going to keep you upright. Body drops one way or the other, and truck jerks. Whereas when a person falls asleep they tend to stay in the position they fell asleep in, hence the classic slow drift to oblivion.
Don't get me wrong, I'd much rather the cause be some undetecable underlying medical issue rather than a lapse in proper judgment. Being in the trucking industry for years I have unfortunately seen that slow drift many times before.