10 year old Female Chessie Suddenly Aggressive

Question
My ex-wife has our 10-year old Chesapeake Bay Retriever who has suddenly begun to display aggressive behavior towards strangers. She growls and nips without any threatening behavior on the part of the victims, and will in fact often switch from allowing them to pet her, etc. to attacking when they start walking away. The target usually is any bag they may be carrying, or she'll grab their hand/arm. She has been a typical Chessie thus far, with moderate dominance characteristics, but hasn't been abandoned or crated or spent much time alone. SO why this behavior, suddenly? Is this sort of behavior common in Chessies?

Answer
The obvious first step is NOT to allow this dog to interact with strangers anywhere, at any time, beginning NOW.

Sudden behavioral changes often indicate illness or developing disability and so the dog needs to see the veterinarian, have a full blood chemistry done, a full examination and a neurological workup.  Having said that, this does sound like a fear acquired behavior which can have developed in three seconds in a manner you would not even have noticed.  if she comes back from the vet with a clean bill of health, repost and we will address it with behavior modification.