It could be one of, or a combination of a few things.
First, if you don't already have one, put a second nock set below the arrow. Nock an arrow and butt it up to the top nock set. Place a second nock set below the arrow and leave a 1/16" gap. The reason is that, upon release, the arrow will slide down the string and bounce off the shelf, making tuning impossible.
Once you have the second nock set on, shoot again. If the problem is still there, slightly lower your nock sets. If the problem gets worse, you were too low to begin with and need to raise your nock sets.
Lastly, if the problem still remains, it could be your form. There have been a couple of recent threads on the subject, and, in both situations, the shooters' drawing arm elbow was a touch high and was effectively causing the index finger to push the arrow down into the shelf.
If adjusting your nocks doesn't solve the problem, take a video or some pictures and post them over in the shooter's forum.