The arrow can't pull the string forward and slap the bow. But you have the right idea. If the nocks are too tight, it can cause the arrow to hit the riser. Your nocks should fit loose enough that when you dangle them off the string, they should hold onto the string, but if you tap the string, the arrow should fall off.
What you are experiencing could also be caused by shooting an arrow that is too stiff. You should research bare shaft tuning, and tune your arrows for your bow.
Since it happens 1 out of 15 shots, one of two things are happening: either your arrows need some work, or your release isn't as good as it should be 1 out of 15 times. Since very few of us have a perfect release 15 out of 15 times, that could well be the cause of your problem.