These rarely happen but nevertheless …
Yesterday, I was playing in the finale of Convocation of Decay when a heavy strike, the kind that a Mauler or Warrior Rat makes, knocked me down. In front of me, there were 2 Warrior Rats but they were not even looking at me or striking. I turned the camera around but there was no enemy behind me.
Previously, I was playing in the finale of The Screaming Bell and on the walkway at the 1st 3 chains. In this location, the enemy can only come from the front (when one is facing towards the center) or from the left if they climb up there. Again, a heavy strike knocked me down but turning the camera showed there was no enemy behind me.
On a different note, vertical strikes from elites which were not directed towards me nevertheless hit me. Again, rare but it happens.