A new contestant has entered the fray -
Look at the slamee's right ear - it doesn't flap, as if a reaction to force, until the slammer's head hits. At least at this speed. It's on giphy and I can't download it and open it in my gif program to slow it down. I might try a different technique.
Edit-- I just did a screencap, then watched it slowly, and I think I was initially wrong in my post above, because t does look like the ear flaps as a result of the slam, but it's too hard to tell if it also flaps as a result of the clash. I'm noting all of this because it the flap was much stronger as a result of the mat versus the head, then I think we could assume that the stronger flap causer was the KO's proximate cause.