There is no singular disease called boxwood dieback, but this does not look like Boxwood Blight if that is what you were suspecting. Volutella is more likely, or that pathogen combined with old winter damage to the stems (this past winter's fluctuating warm-to-cold spells damaged lots of evergreens). More than one issue overlapping each other is certainly also possible, like vole gnawing damage on interior or lower stem bases coupled with some Volutella on other stems. You can explore our boxwood diagnostic page to see if any of the typical conditions they can contract match what you're seeing on still-living growth; diagnosing already-dead and dried stems is difficult and not always reliable because secondary fungi decaying the dead tissues could have colonized it in the meantime.

For issues with winter damage and/or Volutella, the plant can be drastically cut back if the dieback encompasses too many stems to make trimming out the dead zones the only recourse (from an aesthetic standpoint). It will of course take time for shrubs to regain some height after that, but at least they will start fresh with growth that is fairly even and symmetrical, unlike the gaps that can be left from removing only the killed stems on a larger plant. Avoid shearing the plants into more manicured shapes or hedges regardless of their size as this method of pruning creates dense regrowth that increases the plant's vulnerability to pests and diseases.

Miri