Maybe they are getting too much water. Usually yellow leaves are a sign of one of the following too much water, lack of water or lack of fertilizer.

If you live where there have been many rainstorms too much water would be my guess

Not sure about too much the water theory, unless the box has no drainage holes. In a window box, even in a wet climate, plants need supplemental water. The amount of rain falling on that small surface area is not sufficient. Are you saying you have never watered them since they were planted? I would give them a good soak. If they do not perk up then you know lack of water is not the problem.