Hi, 

We think this must be an environment factor. It is too small of an area and in such a straight like that it seems like something has 'burned' the foliage. Was there a car or exhaust from a power machine running and hitting this area? 

Could you have watered and not waited until the water was cool enough? 

Is there a dog that urinates on this shrub repeatedly in the same place? We don't think this could be in since boxwood leaves of waxy and thick so unlikely to burn like turf grass. 

Trying to rule out factors. You can prune out the dead branches to stimulate new growth and keep the plants watered through the heat and drought. Here are some guidelines for watering plants should you find them helpful:https://extension.umd.edu/resource/watering-trees-and-shrubs/

Emily