I think it’s often the seat recline pin not engaged too - on rotation the pilot ends up lying flat on their back.
I've studied the video of the accident many times and although the quality is poor due to the camera and distance away, it does appear as though the pilot's head disappears from view in the later seconds of the video.
Who on the YouTube video says "check .. . check" ... ?!
As a flying instructor it is hard to comprehend that he would not instinctively checked for 'full and free' movement of the controls or that the seat pins were in place by rocking on the seat.
A sad day for sure.