I will say this: although Reeve's horse came to an abrupt halt, Reeve's body (when he was thrown) would have been traveling @ the speed of his horse prior to his horse refusing the jump. The event in which Reeve was competing was cross-country equestrian jumping. Average speed of mounts in this competition is 546 meters per minute (depending upon whether or not the event is preliminary, intermediate, or advanced).
546 meters per minute = about 20.36 mph. The speed @ which Reeve's body could have been traveling when he was thrown from his horse may have been about 20.36 mph.
http://convert-to.com/202/speed-units.html
Reeve wasn't dismounted @ a necessarily high rate of speed.
IMO, it was the angle @ which Reeve's head impacted the ground, and the angle @ which his neck hyper-extended when his head impacted the ground, that led to his spinal cord injuries.
How is this applicable to this case? IMO, the angle @ which MS's head & neck possibly impacted the opposite stair railing and/or the carpeted cement floor below after falling from a 2nd story landing is what caused the hyper-extension of his neck, and is what led to his fatal spinal cord injuries.