It is really unclear, SRS. Some unholy alliance between beetles and fungus. Plus, there are other, more benign fungus that infect the stressed trees and it isn't always easy to tell the benign from the not so benign.

The arborist who took down the big oak was recommended, but I wasn't impressed with his knowledge, and when I did further research based on his comments and recommendations, I realized this was a fellow who had not kept up with the research and knowledge base in his field since he graduated from arborist training, a good 30 years ago.