It could be as simple as them getting too much water with all the rainy weather. Let them dry out for a while and never spray the leaves with water…water the soil below the plants ONLY when needed.
There are also several disease that can attack tomatoes and wet leaves make it easier for them to spread.
Yellow leaves can also be due to a lack of nutrients….scratch some Tomato-Tone food into the soil below the plants right away…you can also use soluble foods for a quick boost as well. Jack’s Classic is the best…Miracle-Gro would be my second choice.