It's also possible if you use too much fertilizer on the plants, they can react like this. In effect "burning" the leaves in a way.
If you haven't used any then of course that wouldn't be the reason. Greenfairy has a good point about watering them too. If you are in a really hot environment you may not be watering them regularly enough.
somul, Water them every day deeply around the roots (avoiding water on the leaves). Also try rotating crops. So next time you plant put the tomatoes in a different part of the garden