My Dad died a few weeks ago at age 89, and he had many of the same symptoms Byrd is showing now. In May, he fainted and fell in his kitchen and broke his femur. He had been more and more "out of it" before that, falling asleep at the drop of a hat, etc. He never really recovered, either physically or mentally. Finally he got pneumonia and died within 24 hours. We later learned that his heart was only operating at 30% capacity.
I suspect Byrd's heart is giving out and he is not getting enough oxygen to his brain as a result.