Yep, it's caused by excess water up take.. So if the plant is stressed from the heat and then it gets watered the plant will take up too much water and makes the tomato split.
little more info. I cut and paste for ya.
One of the more frustrating things about
growing tomatoes is watching a tomato ripen on the vine, slowly growing and changing from green, to pink, to red.... and then looking one morning and seeing that that tomato you've been waiting for has split. Tomatoes split most often just as they're ripening, and, if you have several ripening at the same time, it's likely that all of them will split.
What Causes Tomatoes to Split?
Tomatoes split because of fluctuations in the amount of water they get. If it's been very dry, and then all of a sudden you get a couple of inches of rain, the insides of the tomatoes grow faster than the outer skin is able to, and the tomatoes crack. While this can sometimes be a problem when the tomatoes are still green, it happens more often to tomatoes that are starting to ripen, mainly because the outer skin becomes more fragile the closer a tomato gets to ripening.