It is kinda hard to tell without a few more clues. Does the plant show any outward signs of stress, yellowing leaves or anything else along those lines? Any tracks like a varmint had been messing with the plants?

I do not know of anything that will cause leaf drop on tomatoes without some other noticible physical indications too, like wilting from getting too dry, brown, curled leaves etc. etc.

I am going to guess that it might be birds. Every year, I lose a plant or two to the danged ol grackles. I found

just the other morning. I did not see them do it, but one clue is the little branch that was broken off along with the stem being broken off. I do not know of much that you can do except to replace them with another plant. Scare crows seem to just give them a place to light and survey the garden. They even light on and disrespect ,my fake owl. ":^)
Good luck in figuring it out. See if you can find any other symptoms of clues that you can post.
Bill P.

I am growing Big Boy Tomatoes hydroponically, and the same thing is happening. They have healthy, green stems and leaves, but the oldest growth quickly wilts and falls of once the next set of leaves is established. The plants are less than two months old, so I have 1-2' plants with 2-3 sets of leaves at the top!

I have been looking for information on what could be causing the issue and have come up with nothing.

I have never seen GREEN leaves falling off a tomato plant.

Are u sure they are green leaves? No sign of fungal problems which I HAVE seen cause leaf drop?