There are many causes for yellow foliage on boxwoods. They typically yellow over the winter and then green up in the spring, assuming they are well maintained with water and nutrients. Other reasons could be a lack of moisture, winter desication, root compaction, or severe pruning which can kill the plants.

If your boxwood had a disease, you would be seeing such things as spots on the leaves, dead branches, areas of brown leaves, etc. Many diseases are fungal diseases; overhead watering resulting in leaves remaining wet for long periods of time can bring these on.

In the case of an insect infestation, you would see the bugs or caterpillars (look on the underside of leaves) or such things as blistered leaves, webbing or frass. The link below will help you ID boxwood diseases and pests:

http://extension.umd.edu/resource/boxwood-identify-and-manage-common-problems/

And here is some information on proper pruning of boxwoods:

https://www.gardeningknowhow.com/ornamental/shrubs/boxwood/trimming-boxwood-bushes.htm

Your curving hedge is lovely, and because it is so large and beautiful, you may want to consider consulting an arborist to take a look at your plants. An arborist is a certified and generally insured expert in trees and their problems. He or she can examine your hedge and make recommendations regarding the health of the plants, any needed judicial pruning, and whether or not (in his opinion) any parts should be removed. Hiring a certified arborist will ensure that you are getting a knowledgeable and qualified specialist. You can find a certified arborist in your area here:

http://www.treesaregood.org/

Click on "Find an Arborist" at the top of the page; then click on United States from the pull-down menu. Then enter your State and City. This should bring up a list of certified arborists nearest to you. Or you may be able to locate one under "Tree Services" in your local Yellow Pages.

Thank you so much for the very detailed response… I’m hoping the issue is due to trimming more than usual. I also fertilized the boxwoods yesterday hoping to green it right up..

My pleasure. Good luck!