Sounds like #3 (taken from Southernliving.com:
Boxwood Basics
Follow these tips to keep your plant happy.
By Steve Bender / Photographs: Van Chaplin, Tina Cornett
A healthy, green boxwood looks about as dignified as a plant can be. It adds an air of formality and permanence to the landscape, taking center stage in winter when trees are leafless and then receding gracefully into the background in summer when flowers dominate. Its tidiness and ease of maintenance make it a favorite just about everywhere it grows. But if yours appears more sickly than stately, one or more of the following factors may be to blame.
Poor drainage--Boxwoods can't take standing water and heavy, wet soil. Poor drainage leads to root rot, which in turn causes parts of the shrub to become light brown and die. You can prune out the dead stuff, but unless you improve the drainage by redirecting excess water or amending the soil with lots of organic matter, the whole plant will eventually croak.
Fungus--When a boxwood is sheared to produce denser outer foliage, dead leaves and stems can accumulate, unseen, in the center of the plant. This creates an incubator for fungal diseases that can cause potentially fatal dieback. To stop this, prune back all dying branches to healthy wood (as indicated by the green cambium layer just under the bark). Remove all debris from the center of the plant, and thin out some of the outside growth so that air and light can reach the center.
Fluffy and Fido--When scent-marking pets pass by your plants, yellow and brown leaves soon follow. Repeated dousing can kill entire branches. Applying a product such as Lambert Kay Boundary or De-Fence Dog and Cat Repellent will discourage return visits.
