It's impossible for us to say what has impacted your hollies.  If you put down a herbicide on the area that contained dicamba, for instance, the toxicity may have gone through the soil and killed many roots.  

At any rate, some roots are alive and trying to keep your trees alive. Prune out all dead growth. Cut back to the healthiest growth. Water during droughts for the next 2 years. Hollies can be very resilient. 

Ellen