Boxwoods are susceptible to all kinds of pests and diseases. They tend to trap old fallen leaves in the center, creating a moist pile of junk inside the plant that is an invitation to disease. Thinning the interior of the plant helps to prevent this, and provides better air circulation. Try reaching into the plant and cleaning out the inside. Don't over water these plants either- it leads to more problems.
Down here in Tennessee our boxwoods get Scales on them. They suck the juice from the branches. Take a look at yours look on the branches if you see a "white" looking bump on the branch, its scales. If you push it or squeeze it it will fall off and red fluid comes out of them, flip one over and you can see how they stay attached. You can go to the big home improvement stores and get a Malathon concentrated spray the leaves and branches. it might take a second application. Check them in a week-two for new scales to form. I treat mine usually 2 times a year. Good luck.