I think the proplem is with the head. As far as I remember you were only belted in the, uh… Belt region (if at all, old sleeper busses didn’t have belts), so the head was free to fly up and hit the bed above you or the window. Same thing would of course happen in a seated position, but due to the position, fewer limbs come flying and can hit stuff.

But I’m no researcher, this is just what makes sense to me based on the data provided.