2 Answers

Click here and see sub-heading "Why do tomatoes crack".....

http://gardening.about.com/od/growingtips/tp/Tomato_Tips.htm

Note: Since this is a world wide forum, I removed your email address as I doubt that you would want it publicized for everyone to see. Regardless, we do not respond by email.

Overhead watering & rain seems to be the the most compelling cause that I have noticed. I am sure other factors come into play as well. There are many varieties and some split more easily than others. Some tomatoes like the plumb type tend to have tough skins. Ace, a commercial variety, breed specifically to not split from overhead watering, survive mechanical harversters and ripen all tomatoes at the same time. But backyard gardeners growing gourmet varietals will find many different varieties to choose from. Flavor, sweetness  and applications vary widely. The number of varieties has increased in so many ways that my personal choice for the  Brandywine heirloom variety seems a bit stogy. But the results are reliable.