It's caused by excess moisture, which you've had recently, as you've noted. It's worst when a dry spell is followed by a sudden burst of moisture -- the plant's cells swell up, but the fruit, being hardened by the drought, can't grow fast enough to accomdate the sudden influx, and it splits. However, it can happen even if the season has been moist all along, if environmental conditions are right. Periods of wet, cool, humid weather seem to be the worst for it.

It helps to keep things watered during dry spells, if possible. Otherwise, I don't think there is really a whole lot you can do about it.

Some of it is related to the variety -- some kinds are just more prone to this than others.

Hopefully, many of them will be able to heal the wound enough to continue to develop. I have some apples that have done this this year, because it was very wet and cool back in June and early July. I am keeping them on the tree anyway, because the are an unusual kind, 'Karmijn da Sonnaville,' and this is the first crop, and I want to get as many fruits as I can to sample. They have healed the split areas with corky, brown scar tissue, so I think they'll make it.