Fruit splitting is a common occurrence on citrus in late summer and fall. A single rain event, especially one following a dry period, can cause the fruit to absorb more moisture than the peel can expand to accept, causing the fruit to split. Varieties with relatively thin peels, like some types of oranges, are more likely to split than varieties with relatively thicker peels, like most grapefruit varieties.

To attempt to reduce the problem next year, water the tree perhaps once a week during hot, dry weather and make sure that the tree is properly fertilized since potassium deficiency is known to encourage fruit splitting. See also the section on fruit splitting in this fact sheet

Additional Resource
http://edis.ifas.ufl.edu/HS141