I think it has a lot to do with bad luck, but also with the size, shape, and density of the core. I think part of the reason you often see cracking issues with the same line of balls has to do with how big/small the core is. If a bowling ball is sitting in one place for a long time, such as a ball rack; the heavy dense weight of the core is going to constantly be putting pressure on the less dense filler (some Newton guy said so). I think the constant gravitational pull on the core, when combined with the imbalance created when you drill the finger holes creates some crackage. I think that the filler also cures more and dries out over time as well. Reason being, after you drill a ball, the filler is exposed through the finger/weight holes. If you're not using that ball and the filler starts to cure, with that heavy core pulling on it all the time, it could happen. That's why balls that are used all the time rarely crack, usually it's the ones just sitting around.