It's lag most likely. With the TG patch I've noticed that sometimes when I'm trying to tag someone with resto staff for ult it doesn't go off, and as is the normal behavior during lag, I curse obscenely and keep clicking until it does what I want it to do. When the game finally stops being a scrub it registers all of those light attacks I tried to get off all at once and it looks like my character's right arm is seizing up as he puts out 6 at once. If that's how it's animating on my screen, I wouldn't be surprised if the damage is registering all at once for the opponent.