He's in his death throws and fighting to get the rope. Don't forget this is a former Astartes who is now under the thrall of a chaos god, he's got a bit more strength than the average human. Or alternatively he could be locked in a death spasm. You decide, whichever makes more sense to you.
Lord Commissar gave him a sharp punch to the nerve cluster near his lower back/kidneys, hence paralysis, followed by a short drop and a sudden stop 1800's style.