Well, they seem to rupture around the tops and often in a spiral pattern circling the fruit!
Here you go, all sorts of info on splitting and cracking.
Here is a link that might be useful: Splitting and cracking discussions
I have a lot of watering/rain/weather/growing in containers, issues that I really don't have much control over. I HAVE to use containers, and I HAVE to water them often. The weather I can't control, so the best I can do is keep an eye on my cracking/splitting situations and remove them from the vine asap and let them ripen as much as possible inside. I eat the split ones asap to keep the split from becoming moldy.