Are these older leaves from the bottom of the plant or new top growth? If older leaves it could just be natural death of the lower leaves as always happens.

Otherwise given all the yellowing, likely some over-watering, perhaps some nutrient deficiency, even some early blight.

Your location/zone info on your post would help as it would at least give us a hint as to old the plants are and what your weather has been like - all of which contribute to problems. Also need to know if they are in containers or in the ground?

Dave