That is called cracking. It is caused by too much water at one time. The tomato cannot grow fast enough in size to make up for the volume of water the plant is uptaking. This often happens during protracted rain or keeping the plant too wet.
Yep, too much water in bursts, the plant cannot control itself and pumps the water into the tomato faster than the tomato can grow. If you are watering the tomatoes yourself, try doing it multiple times a day at smaller amounts... but usually this happens when there is a big rain after a week or two of dry conditions.