Why would a passive dog suddenly become aggressive?

My dog is a 5 yr old spayed female yellow lab and is extremely friendly. She is never aggressive or protective to anything. She is a Pet Assisted Therapy Dog and has never barked or growled. She recently has become protective against bones and sticks to other dogs. She just started growling at other dogs, this is... show more My dog is a 5 yr old spayed female yellow lab and is extremely friendly. She is never aggressive or protective to anything. She is a Pet Assisted Therapy Dog and has never barked or growled. She recently has become protective against bones and sticks to other dogs. She just started growling at other dogs, this is my second time hearing it. Why the sudden change? Also how can I get her back into her passive zone?
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