Usually it is caused from not locking your fingers when you start your draw. As you begin to draw, your fingers start to tighten on the string as they attempt to keep the string from slipping off the tips.
Try this, nock an arrow, place your fingers on the string with your hand relaxed. Then begin to close your hand like you are going to make a fist. Bet the arrow just jumps off the shelf doesn't it?
From your previous posts, I know you shoot very heavy bows. Tightening the grip on the string is a common malady with heavy bows. Gotta make a strong hook with your fingers before you begin your draw. Try it and let me know how you do.
Danny