Why Are My Tomatoes Splitting Down The Middle In The Top?

Tomatoes often start to crack during rainy periods, especially if this weather comes after a dry spell. The tomatoes expand too fast. They are most likely to crack when they have reached full size and are beginning to turn color. Irregular watering can also cause this. It is important to water at regular intervals.
There are some varieties that are more resistant to splitting, such as; Early Girl and Jet Star.
Again, it is important to keep the moisture supply as steady as possible throughout the season.

They will do that if, at some point they weren't getting enough water and at another point they got way too much water, they split, watermelons will do the same thing.