What you describe sounds like either a chemical contaminant or a bacterial infection. Was the tree heavily watered suddenly or could there be an underground contamination of some kind? Was there a leak somewhere? Damage to the roots or near the base of the tree could also have caused this.
A "hard pan" or thick, impenetrable layer beneath the tree's roots could have caused a pooling of water that lead to anaerobic bacterial growth and a die-back of roots. I've found it in trees and bushes that have struggled along for years, and then expired from their root problems.
Was the flare exposed? That is one of those issues that can sneak up on an older plant also.
I'm sorry no one answered until now, but if you've lost your holly, perhaps this can prevent you losing anotherin the future, or maybe if it's not too late to save the one you have.
Hope that helps!
Kathe