The fertilizer application probably didn't contribute to the color change, which looks like normal leaf drop (all evergreens do some of this in spring or autumn). That said, you can forego future fertilizations unless the results of a laboratory soil test indicate a particular nutrient deficiency; trees, shrubs, and perennials usually do not need routine fertilization.

We do not see indications of leafminer or mite feeding damage, Volutella Blight infection, or any of the typical boxwood ailments, though you can use the information on the linked page to inspect the plant more closely. As to why the plant on the end of the row is more sparse and off-color overall (orange individual leaves aside), it's hard to say. Root stress is a typical cause of canopy thinning and poor growth, though that could arise from physical damage (roots or trunk base chewed by a vole, for example), over-watering or poor drainage, or under-watering or drought stress.

Did that plant look better when it was first planted? (Were these installed recently, in the past year or two? If so, were the roots well-loosened upon planting?) If it was always the weaker plant, perhaps it arrived with a less well-developed root system or was already struggling with root loss.

For now, there isn't much you can do other than monitoring the planting for watering needs as we move into summer. There is no treatment recommended (fungicides cannot cure existing disease, even if it is root rot), and if it happens to decline further (shedding more leaves, putting out little to no new growth, turning more yellow-green), then it would probably be simplest to replace that individual.

Miri

You're welcome.