It's an new animation of knocking the rider off of their horse, which happened quite often I would think, obviously the animation isn't perfect.
And the couple of second's god mode, is probably something to give them a second chance, because people will start to complain that they get hit off of their horse and had no chance to recover from it. Like it does when you actually kill the horse, the rider fall's off, and sometimes you kill him, other times he get's up in time to block your attack.