I can not be certain this is your issue, but I have seen the same exact thing happen with several guys in my indoor leagues. For almost everyone of them it was they were peaking or watching the shot. They all would unconsciously start looking around the bow to see where their shot was striking and were pushing arrows to the right (they were all right handed).

If your follow through is excellent and you focus on keeping your pin on target until AFTER the arrow strikes the target every single time, then that is not your problem. But if you slip every once in awhile, and usually after you have been shooting awhile, then that MAY be your problem.