Answer: This type of symptom can be caused by a variety of factors. Overwatering could be the cause, however in such small pots I would not expect that to be the case unless the drainage holes have become blocked.
It could be signs of a blight developing, so I would suggest removing the affected leaves and disposing them in the trash. Do not touch healthy leaves in the process, to avoid spreading the problem. Then mulch the soil surface with an organic mulch to try to limit spores splashing back up onto the plant.
Also be sure they are adequately watered (keep the soil evenly moist like a wrung out sponge, never dried out) and fertilize regularly. An overall yellowing and lack of vigor can be caused by low nitrogen. Use a water soluble fertilizer formulated for tomatoes, or use a general purpose slow release fertilizer and/or water soluble fertilizer such as 10-10-10 plus minors or similar proportions. Read and follow the label directions.
Finally, if the plants have grown large, you may need to repot them into larger containers. Crowded roots can stunt the plants. I hope this helps.
Q&A Library Searching Tips