Smith was "behind" the motion of the tackle from the point Mullen left his feet.
He was essentially chasing contact, not providing it. He should have been suspended, as his involvement in the tackle most likely contributed to the fact that the head impacted the ground first, but I also refute the fact that he "slammed" his head into the ground.
As I stated in another thread. The velocity in the tackle from Mullen, and the angle that he was going into the ground would have given a significant chance that his head would have bounced off the ground anyway, causing the same result.... Other than the fact we would all be applauding an amazing tackle by a little man, rather than calling for a suspension.