After much discussion and your alternative question about cheating, I have experienced what you experienced multiple times and I do not think he was cheating.
As others have mentioned there is a ring (the Ring of Thorns) that shoots thorns out at a certain damage threshold. What I have not heard anyone else mention is that there is a second ring that creates an AOE explosion at the point of impact a couple seconds later, the Old Sun Ring. This ring can be stacked with the thorns ring and its behavior would describe exactly what you described. A backstab on someone wearing both of these rings with little physical defense and little fire defense could easily one hit someone with mid to low health.
I do not believe he was cheating, in the countless hours that I have played this game I have not once run into a cheater, not to say that they are not out there. There is currently not a game save editor for DSII for the 360 (you mentioned you were playing on 360 in the other question), I would put it at an extremely low probability that he was a cheater and an extremely high probability he had both rings on and you got hit with damage threshold recoil.