I planted a many different types of plants this year. One was very prone to look like yours. I looked up the variety and found out it was normal. We all had a very extreme summer. It can also be caused when the internal growth is faster than skin growth.
This can be caused by:
1. Overfertilization
2. Extreme fluctuations in temperature
3. Extreme fluctuations in soil moisture
4. Not enough foliage to protect fruit
5. Some tomato varieties are more prone to cracking than others.

Here's a good link. If anyone else has more specific input let me know too.

http://www.missouribotanicalgarden.org/Portals/0/Gardening/Gardening%20Help/Visual%20Guides/Tomato%20Fruit%20Problems.pdf

Thanks for the ideas and link.

too much water at the wrong time.