Not enough water or over watered, check to see if there are any bugs or eggs on there and if so get something that may remove them, Another great way to stop this is to try rabbit poop next year it really works and makes the plant stronger and bigger!! I did some research to possibly solve your problem

Heat and low moisture can cause the edges of the tomato leaves to die back, then twist and curl. Hot dry weather may also cause a symptom called physiological leaf roll. This is a self-defense response, where leaves and leaflets curl slightly to prevent further water loss.

How it is treatable...

1. Select varieties less prone to leaf curl.
2. Maintain adequate soil moisture.
3. Avoid excessive fertilizer, especially nitrogen.
4. Prune indeterminate varieties conservatively.
5. Avoid high temperatures by shading plants, when logistically possible.