I'm still new to citrus trees, and a few weeks ago I noticed a bunch of the navel oranges were split. So I pulled them, set them in a bowl, and got to googling. It sounds like this happens because of uneven watering, they shrink when it's dry, then swell with rain/heavy watering and explode. Just realized today I can use the non-yucky half for cocktails, so that's good! Next thing to figure out is when to pick them... there's still a decent number on the tree (non-split).