Hi - This looks largely like cold damage from the recent frosts. We have seen a lot of cold damage on a variety of landscape plants recently (including boxwoods). Tender new foliage is more susceptible to injury. The clustered sections of browning may be due to Volutella blight, which is a fairly common disease in boxwoods, https://extension.umd.edu/hgic/topics/volutella-blight-boxwood A physical injury (e.g., vole chewing) can also produce dieback symptoms.

Cold damaged leaves will eventually drop out and new growth will fill in as the season progresses. You can prune out the browned areas. A good practice is to thin boxwood shrubs regularly to increase air circulation around the plants-- which helps reduce fungal disease development. Shearing produces a dense growth which allows for increased humidity in the planting (favorable to fungal disease development). You can still shear for aesthetic purposes, but we also recommend thinning. The video on this page demonstrates correct pruning and thinning of boxwoods.
https://extension.umd.edu/hgic/topics/volutella-blight-boxwood 

Here is more information on preventing and managing common problems with boxwoods.
https://extension.umd.edu/hgic/topics/boxwood-preventing-and-managing-common-pests-and-diseases

Christa