Thanks for contacting "Ask Extension" with your question about your boxwood.  
I have a few questions that will help me determine what's happening to your plants.
How old are the plants?
What methods do you use to irrigate?
How often do you water?
Do you have saturated soil in that area during the winter months? 
When did you first notice the dieback?  Did it happen all at once or did it gradually spread?
Looking forward to hearing back from you.

The plants are about 35ish years old and I just use a sprinkler for a 10-12 hr period at night during hot days about 1x/week

Thanks for providing the additional details. I think it's either Volutella blight or 
Phytophthora Root and Crown Rot.
The volitella blight is a fungus. It could have been introduced after winter cold injury. Remember the ice storm we had last winter? Look at the underside of the leaves. Do you see a copper color, or spores? You may need a hand lens to see them.  Also look for loose bark or cancers on the branches. There may also be black streaks on the petiole.(the stalk that attaches the leaf blade to the stem.)

Phytophthora Root and Crown Rot is caused by water molds that cause root or crown rot. They thrive in saturated wet soils brought on by poor drainage or irrigation methods that prolong water saturation. Look at the roots of the affected plants. If the roots are bad looking (chocolate brown to black), that is a pretty strong indication that you may be dealing with root rot. 
To get a positive id, you can bring in a sample (including the root) to the Master Gardener Help Desk (address included at bottom of response), or for a fee, you can submit a sample to the OSU Plant-clinic.  https://bpp.oregonstate.edu/plant-clinic
I recommend you prune out the disease portions of the plant or remove the disease plants completely to avoid continued spreading. If the plant has Volutella blight, pruning might help the plant to recover. If it's root rot, the plant will most likely die anyway. 
To prevent future infections  switch from overhead sprinkling to a soaker hose or drip. Avoid over watering.  Add  2-3 inches of mulch to help prevent water saturation.
Hope this helps.  

Extension office/ Master Gardener help desk:

Sunset Building, room 107
4077 Research Way
Corvallis, OR 97333

Thanks - I’ll give them a good trim and see if I can tell what’s going on - they need it badly anyway- thanks for your help!
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