I've had a few of them split on me too. It's caused by pressure inside the crank case. The pan flexes up and down until it fatigues and splits along one of the creases.

You're probably getting a lot of blow by or don't have the engine properly vented.

We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon