I did that to my friend once, i didn't knock em out my friend did that at me just playing around during an arguement and i stuck him in the gut hard enough to knock the wind from him.
I think that guy just didn't know what the **** was going on after the first batch of headshots. Which is why he kinda stumbled over, thought he should hit him, then didn't. I'm pretty sure he was unaware of what was going on, and was just confused from his battering.