Same thing was happening to me. Then only sporadically and it wasn't until then that I realized that the only time it happened is when I was overdrawing. Now I pull right to the corner of my mouth and it almost never happens. When it does I know I did something wrong. Oh, and my shooting improved greatly when I stopped trying to muscle the bow back too far.
Another thing, and this took me months to 'feel' make sure your drawing hand is relaxed. If I pull the string back with my hand like a claw and all tensed up, I will most likely twack my nose.