Probably a flaw in the manufacturing of the ball. something foreign introduced into the plastic, whatever. No big disaster when it happens to a pool ball. Now I'll scare you a bit. Counterfeit fasteners (nuts and bolts) are on the market. This means fasteners not up to their strength ratings. These are being discovered everywhere, even in the aircraft industry. A Porsche engine builder told me of rod bolts failing...bolts torqued properly using a stretch gauge and purchased from a Porsche dealership as the real deal. He took the bad bolts to be tested...yep, counterfeit, even though they came out of a sealed Porsche factory parts box. Just something to think about as you're sitting in an airliner seat, waiting for takeoff.