Since the crack goes around the ball, it sounds like it's ready to go to that bowling ball graveyard. The cover probably continued to cure over the time period, or possibly the core expanded. As we use these balls the lane conditioner helps keep the covers more pliable. If they sit too long, even at room temperatures, they can crack. There are other possible factors, as charlest suggested.
Every ball that has ever cracked on me was sitting on a shelf in my office, but in all but one case, I hadn't thrown it for awhile. One time I actually heard it crack, which was weird.