I'm not an expert on tomatoes, but I once gave my cherry tree and grape vine a regular watering and both the grapes and cherries split. Someone told me that this was caused by too much water being taken up by the fruit which caused the fruits to swell and split. Maybe this is what's happened to your tomatoes and someone else could confirm this?

Yes that is one of the reasons, but also they will do this if they are let to get dry and then have a flush of water, try to keep them wet and don't give too much water at once as this will be a cause of them splitting, this is one of the reasons that I found out myself anyway.

The warm days and cool nights at this time of year have an effect too. The skins have firmed up and then split as they pull up the water

The cause is, as the others suggest, irregular watering, but also fluctuating temperatures - we've had some heat and some cool, and that doesn't help. If they're in a greenhouse, shading and cooling it helps, but otherwise, the only thing you can do is to keep them well watered from the moment the fruit starts swelling.

Yup irregular watering...