Certified GKH Gardening Expert
Unfortunately, there are quite a few causes for this. With limited information and without seeing the damage, I can only guess.
Bacterial/viral infections can cause this, and is most common with overwatering. Tomatoes need to dry out some between watering. At least 2 or 3 inches below the soil should be completely dry before watering again.
Many nutrient deficiencies can cause leaf drop, as well. It could be that the the plant has used up all of the nutrients available.
Last, it could have received a shock of heat, or dried out completely before being watered. This can cause significant leaf drop.
These two articles will help: