Tomatoes are susceptible to quite a few foliar fungal diseases which can cause leaf yellowing. Usually the problem starts as yellow spots, but eventually the spots turn brown. You can spray with preventative fungicides to keep the problem from becoming too severe.
There are also soil-borne fungi that can cause leaf yellowing on tomatoes. Both fusarium and verticillium wilts can reside in the soil and be picked up by susceptible plants. There's no cure for these vascular wilts, but many varieties have good resistance. Look for varieties with V or F after their names, indicating that those varieties are resistant to Verticillium wilt (V) or Fusarium wilt (F).
Black walnuts can also cause similar symptoms. Avoid growing tomatoes near walnut trees.