Border collie bites when you leave

Question
Hi, we have a 3 year old, male, neutered, border collie called sammy. He is a rescue dog, and we think he has had a past of being abused, because he is very insecure.

When we go through our passage door to leave the house, he runs at us barking and occasionally nips us. But my grandma lives with us too and when she leaves, he runs at her and actually launches himself and goes to bite and now shes scared to go out of the door.

We had a home visit from a professional dog trainer, chriss briggs, who has border collies himself. He said sammy isnt agressive just really naughty.

But as sammys really intelligent he was on his best behaviour while the trainer was here. So chriss didnt see the real him.

He is a really nice loving dog who takes treats as gently as possible, but we just have that leaving the house problem.

Do you know why he does this, is there anyway we can stop him?

Answer
A dog trainer isn't qualified to diagnose a complex problem such as the one you describe.  A full case history needs to be taken, an expanded version of what you just wrote, and your trainer (no matter how talented) does not appear to understand that a diagnosis and treatment can be made without the necessity of provoking the unwanted dog behavior.

The Border Collie is a driven working machine whose aggression is truncated from "kill" to "herd"; the "nip" you describe is part of the process of herding.  The Border Collie (especially the male) is often a candidate for misunderstood "aggression" in behavior due to the strong drive to control the "flock" or "pack" in his household.  Your dog should not have run of the house to the extent that he is able to control who leaves, how they leave, etc.  He is definitely feeling the insecurity of separation and is using his genetic predisposition to control in order to take charge of a situation he cannot possibly, ever, manipulate.

Immediately put this dog on a Nothing In Life Is Free regimen.  He must earn everything: his food, his water bowl, being allowed out or in, being petted, etc.  Find a room where he can be contained before you leave the house -- well before, at least ten minutes.  Examine one another's "leaving rituals"...and change them so the dog cannot anticipate what will happen next.  Randomly put the dog in his containment area (kitchen or bathroom, with special toy or treat) during the day for varying intervals (one minute, ten minutes, 30 seconds) so he cannot connect your leaving the home with his containment area.  If the dog sleeps with you in the bedroom, change that immediately.  You must psychologically demote this dog so that he perceives himself as last "in line" in your "pack".  When left alone with your grandmother, the dog must be contained and unable to attempt to control her.  He obviously perceives himself as much higher in "rank" and, being afraid of him, she is worsening the situation (inadvertently, of course!)

Your next step is to go to Karen Pryor's website and find out how to use a clicker to train your dog.  Teach him some obedience "tricks" using made up words (not the standard, "sit", "down", "come", etc.)  When you have 100% compliance to one or two commands (with frequent short training intervals over approximately three weeks, you should begin to see this result), buy three large beach balls (non-puncture type) and begin to teach him to herd these in the back yard or a safely confined space.  Your dog needs to WORK, that is what he was designed for.  You can also teach him to "find" articles in the home by hiding cookies in plain sight and clicking as he gets to them, then hiding them a bit more diligently, making his work a bit harder.  Give him a JOB every single day and prevent him from attempting to pull "rank" on your human pack members ASAP.