Makes me wonder what happened that caused a behavioral change like that? Someone has done something to trigger a trust issue. Dogs typically don't just change like that without a cause. Has it been sick? I knew of a dog an acquaintance owned, that got lost once for about 2 weeks. It came back, but was just different. Mean different. They took it to a vet and nothing seemed wrong. He said the dog would act alright. Could be petted, fed, watered it was fine. Then next thing you knew, you'd start to pet it and  would growl and bare it's teeth, even try to get a hold of you. Get very territorial over it's food and personal space and then as quick as it turned , it would calm down. Sounded like PTSD to me. Made me wonder, if a neighbor of theirs hadn't took it. Maybe mistreated it until they realized it wasn't going to stay with them, so they cut it loose.