I think its due to the weather and uptake of water.
It seems to me that after it has finished growing and reached its size, then beging to ripen the skin is not flexible enough to stretch.
Hot and cold days so uneven uptake of water seems part of the problem too. My small toms seem worst affected. I put them in salty water then rinse before I eat them and pick tnhem as soon as I see the split

It would seem that a lot of gardeners are having this problem this year as Pamg says it could be the uptake of water, do you water them on a regular basis.

Many thanks for helpful tips on my 'splitting ' tomatoes... They do taste wonderful despite the splits ... I have also not had the problem with my other varieties.