This happens when an enemy is alerted of your presence before you backstab. Since they were alerted (You punched them) so they spun around after punch animation was complete, but then you had used the exo-knife, which put you behind the enemy. The enemy was always alerted to you, so they spun to face you, except them turning turned you, so it was like a dog trying to bit their tail (Corgis are a funny example.)