More than likely to much water ... or too little !
What I would do is get ruthless with them. Get a pair of scissors and snip off all the lower limbs, right up to the first flowering limb. This will open up the space between the lower stems and the ground. It is across this area that disease will transfer across easily. Also remove any straggling limbs that interfere with other plants. There is no point in having big green bushy plants, removing the excess limbs drives the growth into the fruit.
As for watering .. I water every day in the morning.
The same amount of water is given each day. Dont give them extra because it is warmer one day than another.
Consistent watering will give you a better product. It will stop you getting a watery tomato and it will also reduce them from splitting.
Whatever you do don't let water land on the leaves as your water them, make sure you only water the ground around them.
Feed .. only once a week when the flowers are set .. not before.
After tidying the plant up should the leaves seem discoloured, just give them a feed of Epsom Salts every other week. Just add 2 tables spoonfuls to a watering can and soak the base of the plant.