I had this happen to my Tango mandarin really badly last year. The explanations I found were:
1) inconsistent watering - peel becomes dry, then fruit swells quickly and bursts it. This happens very easily with tomatoes
2) too much fruit set. The mechanism wasn't explained, but my Tango did have a massive fruit load and at least a quarter of them split like this
Not sure if that is your cause, I haven't ever seen this with my pomelos.