Splitting or cracking of tomatoes growing on the plant is quite common and is often caused by fluctuations in soil moisture. This can be caused by the grower when watering too heavy after a long period of not watering. It can also be caused naturally when there is a period of dry weather followed by a period of heavy or above average rainfall. Splitting and cracking usually occurs in the later stages of fruit development, just when the fruit starts to ripen.
How To Prevent Tomatoes From Cracking
Watering - The best way to prevent tomatoes from splitting or cracking is to practice a watering method and schedule. It's best to water tomato plants deeply on a regular basis that to provide only sporadic, light waterings. That being said, be careful not to overwater tomato plants. They like a moist but well-drained soil. Consistently wet or soggy soil will most likely cause other diseases. Adjust the amount of watering you do to the amount of rainfall received. A good natural soaking rain usually lasts longer than a hose watering with treated water, unless you are watering with natural water from a well which would be equivalent to a natural rain. When "determinate" tomatoes start to ripen you can greatly reduce the amount of watering to reduce the chances of cracking. Determinate tomatoes are ones that produce heavliy all at once - at a "determined time."
Feeding - Another thing that can cause tomatoes to crack is over feeding when they are starting to ripen. I use a slow-release, mild organic tomato food to fertilize tomato plants. When fruit starts to ripen I usually discontinue fertlizing, especially on determinate varieties.
Crack-Resistant Varieties - Also, there are some tomato varieties that are "crack resistant." You could look for and plant these varieties. But, still, follow regular watering practices and avoid over feeding.
Earlier Picking - Pick tomatoes early and allow them to ripen indoors. If you pick tomatoes right when you see their skin start to crack, they are still okay to eat. If the tomatoes are left on the plant and the cracks are severe, to be safe rather than sorry, I would suggest not eating these. Instead, remove them from the plant and discard to avoid attracting insects and onset and spread of disease.