It's directly related to the variety you grow. I don't know if growing conditions have any influence. But a thinner-skinned tomato is more likely to split. I have been growing Romas for the past few years because they seem to be a happy medium between skin thickness and splitting. This year I also planted Heartlands and (I think...) Manitobas (I lost the tag) but they haven't ripened yet so I don't know what the skins are like.

Good point about the thinner skinned tomatoes splitting. The only ones I'm eating so far this year are a cherry-type tomato and I have three or four other varieties turning pink, salad slicer types. I'll make notes on which ones have thinner skins but don't split, and will ask at the nursery. Thx!