User blog comment:Emptylord/Leaked Champion: Vel'Koz/@comment-5788083-20140120111422/@comment-3974211-20140122153917

It's probably because the knocked "shifts" them in a set direction, as if the ground were moving beneath them (Tectonic Disruption). Things like Blitzcrank's hook show the stun animation just to illustrate that things cannot be cast.