Answer: Basically, tomatoes "crack" when they grow too fast. It can happen if lots of water is applied after a very a dry spell (meaning uneven moisture levels) or it can happen during a period of lots of rainy days which make the insides grow faster than the skins can keep up with. Sometimes the cracks heal over and other times they become the site of other infections which can make the fruit worse than just ugly. (Generally, cracked tomatoes without other problems are fine to eat.)
Some varieties such as 'Burpee Big Girl', 'Celebrity', 'Husky Gold' and 'Sweet Million for example, are less susceptible to or more tolerant of cracking than others, so you might look for them next year. You might also try using a mulch and keeping an eye on the watering so that the moisture levels are fairly constant as tomaotes do best with a moist yet well drained (not soggy) soil. A soil well-enriched with organic matter also tends to help the plants weather seasonal moisture fluctuations better.
Good luck with your tomatoes!
Q&A Library Searching Tips