Bigcountry,
I'm not an expert, but it sounds like you had a sharp edge in the nock. Happened to me once! Or, you had a tiny fracture in the nock, and the pressure on the string sheared it's way in and cut the string,,, One way to help prevent this is to rub nock grooves with a Q-Tip, or cotton swab. any snags or rough spots will really show up!! "bowstring"