Same sort of thing happened to me but in PvE. Prison 35, the round before Skolas. I was back in that left corner side, up the ramp sniping. Suddenly I flew backwards, all the way to the wall, which I hit full force and died. It was almost like what happens when you get whacked by the Cabal Phalanxes, except we were fighting Fallen and there was nothing anywhere near me. Still, hilarious.