Are they professional dog walkers? Meaning, do they have years of experiance with many dogs and many (different) dog behaviors?
Taking your post on face value, your normally well behaved non-aggressive dog, suddenly bit the well known handler twice, although unprovoked and with malice. Without previous warning or indications of aggressiveness.
If it were me, I'd speak face to face with the walkers. I'd ask many questions and take notes. I'd want to know exactly what the behavior of my dog was from the moment they arrived till the moment they left. I'd want to know what was the behavior of my dog last week (etc.). I'd go to the vet well armed with any information provided.
Also, don't get hung up on 'thyroid' issues (although a good place to start). It could be anything. My sister's dog suddenly bit her one evening while they were sitting on the couch. This dog has no aggression issues. I suggested taking her dog to the vet. Turns out the dog's anal glands were impacted. The dog was in serious pain. Not giving the dog a free pass on a bite, but certainly explains things.
Thanks for the feedback Lilie. We have a face to face meeting with the dog walker tomorrow evening ... they aren't available tonight. They are seasoned dog walkers with almost 10 years experience. We'll have to ask about their experience dealing with dealing with behavioral issues in dogs.
In talking with them today, there have been no issues with Jake recently - no other changes in behaviour. They were in our front yard with no one around. Jake did his business and when finished he jumped on them and bit him in the arm. I asked if they could think of a trigger and there wasn't any they could think of. He continued to be aggressive going into the house and he couldn't get his leash off of him because he kept trying to bite. I asked if it was possibly over excitement, but it displayed as aggression according to them. This is very out of character for Jake. He's never shown any sign of aggression in the 8 months we've had him.
I'm not sticking to just the thyroid issue tomorrow at the vet. I realize it could be any number of things if its medical related.
I'm taking this incident very seriously and this is not ok. It could have been one of us, our kids, friends, family or a stranger the next time (but I pray there isn't a second time). If we have a medical issue, lets deal with it. If it's the beginning of a behavioural issue, we need to know as soon as possible and start dealing with that.
As an additional note, I am home now and not noticing any abnormal behaviour with Jake. I made sure I had lots of treats on hand to re-inforce in training. When I let him out of his cage he was his usual excited self, but calmed as usual for me. We did a short obedience session and ran in the back yard. I felt him all over and didn't seem to get any response or he wasn't sensitive anywhere.