What Causes Tomato Leaf Curl?

It is basically due to the sun light. Every plant needs to get sun light. And for this reason the leaves of the plant would turn towards the sun. From your question it is clear that you've planted your tomato plant in a place where sunlight is not proper so the leaves curl themselves to get that.
It is the natural process of developing first the fruit develop slowly but at the time of the season you'd find it totally developed.