My friend Ross suggested diet as a contributor to excessive aggression.
I suspect it is possible, but I don't think it would apply in Doc's case. We feed our dogs a home-made mixture: fish, grain, vegetables, and a bit of olive oil. Maybe the fish is mercury-laden. I usually use canned mackerel. It's possible. I've heard of people who eat excessive amounts of sushi can develop hints of mercury poisoning.
There are some good commercial dog foods out there, expensive, but mixed from good quality ingredients. I find it easier to mix my own, and I make enough at a time to last three to five days, storing it in the refrigerator. I do know it has improved our dogs' health greatly, and it has cut down on waste.
My wife and I have watched Doc more closely over the last week or so, and we are now theorizing that his increased territorial drive may relate to our granddaughter who lives with us. She'll soon be eighteen months old, and in the last three months or so, she has changed from a baby to a little person -- talking, more mobile, more demanding, interacting with the dogs more. We think Doc, tender, apprehensive soul that he is, may have gone into protection-overdrive because he senses the little girl needs protection.
It would be an interesting project for an animal behaviorist, I suppose.
I do know when I went to craigslist to look for someone who might need a miniature guard-dog, there was more than one person there attempting to find a home for a dog because the animal wasn't able to tolerate a new human baby.
Of course, this is the reverse -- Doc may feel so conscientious about his territory and the new person in it that he may be overstressed.
For what it is worth, the local craigslist is overwhelmed with pet give-aways, so much so that there was an person from the local shelter saying "Don't bring 'em to us" because the situation was so bad that the majority of dogs the shelter takes in are euthanized.