Sounds like the splitting might be due to inconsistent watering. For example, letting the ground dry out too much before watering. Or maybe it was due to too much rain at one time.

Rodney

Ah, makes perfect sense. Now that you say that the few times I've had them split was after some heavy duty rain! Thanks!

My red cabbage does the same thing after a big rain sometimes - just go ahead and harvest it. Don't wait for the insects to move in.