Unfortunately there's so much dieback that it's hard to tell what the cause was, although more than one issue might be overlapping at this point. Possibilities include severe winterburn (damage many evergreens sustained this past winter), an infection like Botryosphaeria canker (brought on by environmental stress like drought, lots of pruning, etc.), some scale insect feeding (not causing dieback per se, but we see what looks like some sooty mold on the green leaves), and dieback of the tender new growth from the overnight freeze back in late April, which damaged many plants that had already broken dormancy and begun leafing-out for the season.

It will probably be more practical to replace the badly-damaged hollies, but if you prefer to keep them, you probably would need to renewal-prune them, which cuts down all branches to within a foot or so of the ground. Several evergreens, including hollies, respond well to such measures, though it may take a few years of regrowth to regain their former fullness and size. If you try that approach, now (spring) is a good time to attempt it. Either way, even if you only prune out dead growth for now, monitor the shrubs for watering needs (see the linked page for guidance) as the ongoing drought is stressing plants, and the production of new growth requires readily-available moisture in the root zone.

Miri