This can be a common phenomenon for Japanese Hollies (and some other holly types), though a precise diagnosis is hard to make with certainty from the photos, as the symptoms are not very distinctive. Fungal infections like, but not limited to, Web Blight (Rhizoctonia) and Thielaviopsis root rot can both affect Japanese Holly and cause branch dieback. Depending on how you have been irrigating, if foliage has been getting wet from a sprinkler, that might increase the risk of Web Blight infection, and if the roots were being over-watered, that might increase the risk of Thielaviopsis infection. Other conditions, like soil that is not acidic enough for the preferences of hollies or over-fertilization, can also predispose roots to Thielaviopsis infection.

In either case, use of a fungicide is not recommended, nor would it be able to cure existing disease. We can't rule out drought stress as one possible factor behind branch wilting or dying (either directly, or from infection by opportunistic fungi like Botryosphaeria), whether from this growing season or last year, when we also experienced pronounced drought. You mention watering the plants, but we don't know how much water is being given and how often. The ideal is to water thoroughly only once the soil becomes somewhat dry to the touch about six inches deep; if damp when checked to that depth, watering is probably not needed. As a very rough estimate, about 3 or so gallons (per shrub) would be needed to re-moisten the entire root zone when watering was needed.

Is lime applied to the nearby lawn yearly? You said the soil test was fine, so we presume the pH was acidic enough? (Close to 6.0 or a little below?)

As you noted, all that can be done for now is to prune off dying branch tips and continue monitoring the plants for watering needs. Thinning growth overall could be due to a range of factors, including soil moisture and drainage, differences in soil compaction, and whether plants on that end of the row receive shade from a nearby tree or building. Do not fertilize unless the soil test results showed a significant deficiency in certain nutrients; established trees and shrubs do not need routine fertilization, and plantings next to lawns that are fertilized yearly likely receive plenty of supplemental nutrients already from roots infiltrating the lawn or water runoff that carries those nutrients into their root zone.

Miri