Tomatoes split due to fluctuations in the amount of water they get. Whether a heavy rain has fallen, or the tomatoes were lacking water and excess water was used as an attempt to make up for this, cracked tomatoes are commonly the result.
Tomatoes that crack are the result of the inside growing faster than the outside can handle. When the inside of the tomato grows rapidly, usually due to excess water, the skin on the outside is not prepared for the excessive growth and splits to accommodate it. Cracks can happen to tomatoes that are still green, but it is more common in red ripening tomatoes because the outer skin is more fragile as the tomato is ripening.
Look for varieties of tomatoes that are resistant to cracking to prevent this problem in the future. Other ways to prevent split tomatoes is to water regularly, even during rain. Mulching reduces splitting as well. Two to 3 inches of mulch around a tomato plant helps retain moisture in the surrounding soil. Eating tomatoes that have split is OK, but once they split, they will not be able to be kept as long as tomatoes that do not have a split in them.