Why did the temperament of my grandma's Border Collie change?

Ok, this is a dog that has always been soooo sweet and happy and eager to please. We have taken him to the groomer, the vet and stuff before, and he has NEVER been the least bit aggressive (just a major leash puller). But, I took him the groomer the other day with my grandma (his owner) and he's had at least 3 professional groomings before so it wasn't a brand new experience. He snapped at the groomer after we had been there for only one minute. This groomer was just asking us what we wanted done, and just ran his hand thru the fur to check the length. Then a different groomer actually did the grooming and at the end SHE said he tried to snap at her several times during the grooming! (He looked great though so we gave her a good tip for her troubles lol).Then at the vet (the same day) the female Vet Assistant tried to listen to his heart/lungs with a stethoscope and he tried to snap at her too!!! Then when the vet came in (a man who has seen the dog since he was 12 weeks old) he rubbed his ears and it all good. Then I am not sure what happened, but the dog got scared again and snapped, and then tried to snap again!! It was starting to creep me out because this is COMPLETELY out of character for this dog!!!Vet went to get a muzzle and I put it on the dog (with no trouble, the dog trusts me). I tried very hard to calm the dog down (while staying calm myself) and make him feel comfortable but it didn't work.The dog is about 3 years old. Purebred. Lots of energy of course. The dog mostly stays outdoors in their HUGE yard. Sometimes there are sheep back there, sometimes not (grandma lets a relative house his sheep there when he wants). The dog is never walked on a leash (I've tried in the past with some progress made, but since I don't live there to do it daily he reverts back to PULLING the leash). June of 2008 he weighed 50lbs (as a full grown adult dog). June 2009 he weighed 62lbs! The weight gain mostly happened over winter and it was definitely noticeable. Grandma gives her dog way too many treats. Vet talked to her about it and I reiterated everything.BUT WHY? Why would his behavior change so much? Grandma has had this dog since dog was around 12 weeks (maybe even younger). Dog is around 3 years old now. Never acted like that before. What set him off? Was it just a bad day? Too much activity in one day? (But the snapping started right off the bat so I don't really think that was it). I'm afraid that he will hurt strangers that come by her house now and that maybe my grandma will either get sued, or worse have to put the dog down! But originally I never thought that dog could hurt a fly!!

The dog has not been socialized properly. 3 groomings in his lifetime is next to nothing. He could hurt somebody, and is a liability. Is he neutered? That should be the first thing. Then work on socializing him properly. Enroll him in some obedience classes. He is coddled by the grandmother and thinks he's the boss. He needs to be taken down a notch and put in his place as a dog and not the master of the house. He will be a much happier dog if you do this. And I do not mean by rough handling at all, only positive training methods need to be used. If you try to roughly train a dog already showing aggression, you will only get more aggression out of him.

I have gathered several links here that might help you out. Good luck!http://dogtime.com/aggression-in-dogs-aaha.htmlhttp://blogs.dogtime.com/go-dog-training/2009/04/question-teaching-leave-ithttp://dogtime.com/dog-suddenly-aggressive-toward-dogs-wild-faq.html

Several things that could be going on- and I suspect that multiple issues may be interacting to produce this.Since he's never walked on leash, he doesn't get socialized, doesn't meet new people, dogs, etc. So he doesn't have a recent history of being good with people of any sort, much less people who are doing things to him. Even though he may have been well socialized as a youngster, if it isn't maintained, it gets lost, especially when the dog reaches social maturity- usually around three years. Pups who allow anyone to do anything to them don't necessarily stay that way if that isn't specifically worked on.It is possible that when he was at the groomer last time, he had a bad experience- this is actually fairly common. Dog could be in pain from any of a variety of sources, especially since he is overweight which would contribute to painful arthritic and/or dysplastic joints. Lyme disease and other tick borne disease often go unrecognized, and can cause significant pain. There are dozens of possibilities of why the dog could be in pain.So now you have a dog who may be in pain, who had a bad experience before, and is now socially mature and doesn't feel the need to tolerate what strange humans might be doing to him, who bites in fear of more pain. Not unreasonable from the dog's perspective. First thing would be to diagnose the cause of the pain and get that treated, do some counter-conditioning to being handled, and get him out and about a bit more.