Nearly all of Maryland has been in some level of drought status since spring, so there really hasn't been an excess of rain, even if some areas received occasional showers while others did not. If the plants were accidentally over-watered (which would probably take a lot in a drought year) then root rot might be responsible, but in this situation, we don't think that's likely.

Yellowing foliage to this degree does suggest some sort of stress, though, but it's hard to tell what the cause was. It might be drought if the shrubs weren't being watered occasionally, or root damage from some other cause (any voles causing problems in that part of the yard?). A Volutella Blight infection might have set in at some point in the past growing season or so and only now is causing stem discoloration, but if that is the case here, nothing can be done except to wait and see how the branch fares, since fungicide would not resolve the issue. If those yellowing branches do die back (leaves will turn brown and brittle, even if they don't fall off right away), then they can be pruned off. Our boxwood diagnostic web page has examples of common ailments and (when applicable) their management, including for Volutella.

Miri