Aikeway

I've often wondered why Tomiki Sensei classified ushiro ate as atemi waza. At first I thought it must have derived from an additional knee strike into the back, or throwing uke onto a knee placed behind uke as sometimes seen in Daito Ryu techniques. However, that can't be the explanation as the atemi waza must be to off-balance uke rather than an added atemi after the throw. In my experiments, ushiro ate seems to work better as a grab on the shoulders rather than a strike or double strike on both shoulders to effect the off-balancing. If the technique derives from a strike on the shoulders, was it a double palm heel strike or a knife hand strike? Would anybody have any knowledge on this?