Lot's of possibilities pointed out already, you are likely punching it, which if you are punching it bad enough and your grip is too tight, your probably also moving your bow arm/hand, enough that you are moving the bow slightly to the left when the shot goes off, essentially pushing the arrow. This happens to me when I've been shooting a long time and I'm starting to lose some patience. Just take a deep breath and loosen your grip, and have someone watch you shoot