[Winston]

Your dog is biting for a reason especially if this is not a common occurance. I would suspect some form of pain. Perhaps considering age she may be arthritic but honestly its so hard to say without a medical check up. I can think of a whole lot of things that would make a dog groutchy.

As for you perhaps its because you are the leader..They dont tend to do that to those they respect if that makes sense. They will if its fear biting etc but it sounds like she doesnt like being disturbed.

I would then look at each scenario and see if there is anything that is common about it.

For example what if she was loosing her hearing and someone approached from behind to startle her...dogs learn sounds and maybe she is used to how you sound when you walk but others not so much..

Sounds crazy I know but hey you just never know...keep us posted please would be curious if you get some resolve..