We have an 18-month old Papillon. He is generally well behaved and learns things quickly. The problem is that about 8-months ago, another cat was bullying our cat to the point where he was afraid to leave the house, and on 3 or 4 occasions, we let our dog out the back door to chase this cat out of our garden. He barked at it and it ran away, and it no longer visits our garden.
But since then, he gets very excited if we go near the back door. If we go outside (which we do often - our washing machine is in an outhouse) he rushes out of the door, barks and jumps up at the wall (where a cat could climb in the garden), and then runs round to a tree, climbs a little up that (it forks about a foot off the ground) and barks some more. After he's done that for a minute or so, he has one more bark at the wall, is satisfied and comes back in.
He's got gradually more obsessed about this. If he is upstairs and hears one of us step near the door, he comes tearing down the stairs at full pelt, sometimes yelping as if in pain if he thinks he might not make it. As soon as we get home from a walk he rushes to the back door and looks up at us expectantly, sometimes barking to let us know he wants to go out on his little routine.
We've tried ignoring it, but we still have to go outside most days and he always wants to come and do his thing. If we shut him away from the door and go out he whimpers and yelps and generally gets rather distressed. We also tried indulging him and taking him out on a "security patrol" when he wants to, thinking that normalising it might take the excitement away. Neither thing has calmed him down.
There is a cat flap in the door which he uses without any fuss when he wants to go outside for any other reason (e.g. he needs the toilet in the night, or he just wants a quiet wander round the garden), but he doesn't choose to use it when he wants to go and bark at the wall and up the tree - that seems to specifically need us to be involved.
Any advice on how we can wean him off this obsession?