Splitting is usually watering-related, but it's inconsistant watering more than too much or too little. If you forget to water for several days and then overwater to compensate, or a big storm comes through, you're likely to see some splitting. Try to water regularly and adjust the schedule when it rains a lot (or not at all).
You also can pick tomatoes when they start turning red and let them ripen inside rather than leaving them on the vine longer. The longer they're on the vine the greater chance you'll see some splitting.