Why is my dog suddenly aggressive towards select people?

My 5yr old neutered basset hound is the sweetest and friendliest dog. There have been incidents where he has growled at select people, some which have returned, some were strangers and he would tuck his tail, lower his head and slowly walk away. We didn't think anything of it until recently when it went too far. My sister told me that today, he was growling at her friend who had spent the night (he had been fine with her all day). They were sitting on the floor and he hid behind my sister and continued to growl. The friend tried to baby talk him, trying to change his mind and maybe get a tail wag but he wasn't having it so they left him alone. Later, he started up again and this time lunged for the friend as my sister held him back. The friend then ran out of the room and he chased her supposedly with intentions to bite. I have no idea why this happened and my parents are threatening to get rid of him. Please hep for my best friend's sake!