I have an older relative staying with my immediate family. At first he and our dog got along fine. It was when my relative became very ill and subsequently required temporary hospitalization when my dog started growling at him. I don’t know if the dog is picking up on some instability or something like that.
I don’t know if it matters but the dog’s breed is mixed but mostly a Golden Labrador and has been a wonderful family pet. The relative has late stage liver cirrhosis and many of the complications that come along with that type of disease.