Hitting people from the back in any conext is totally illegal as far as I’m concerned. Especially when the first contact by the aggressor comes from some part of their body above the waist. In my experience I’ve had guys who were in a full sprint come up behind me while I was jogging in the box looking past the goal to where the ball is, and they throw up a forearm or elbow to decelerate themselves using my spine as a cushon. In these situations you get direct contusions from the initial blow, and sometimes whiplash and cervical injuries because you can’t see it coming. I think when any adult does this it is borderline assault, and should not be a part of the game at all.