It was a very slow process. Nothing you would pin point as the "culprit" by itself.
Lets start at the beginning with what Scooter was like when we first got him. We found him in the parking lot of Chili's. Yes a stray. Full grown but small for being a border collie. We left and went to the humane society had them take pictures and check for a microchip. We had decided to house the dog until we could find his home. We put an ad in the paper and traders. We then brought him home it meet our two other dogs. He was and still is a great dog.
No one ever came forward to claim the dog and so we kept him. He soon got a name Scooter and became fast friends with our dog Zippity an Italian gray hound. The two of them would race and they found that if they worked together they could catch and eat wild rabbits. (it bugged me)
Because of being so physically active neither dog was over weight.
But when Zippity left us :( about eight months ago. Scooter lost his best friend. Sure we still had another dog Banard a corgi who isn't at all interested in chasing anything.
So there is his start to obesity.
Then a few months later when leaving the house to take the kids to piano lessons we found a COW tail that the dogs had. Had they found a dead cow? We never found out, but I know they were eating something gross that I took away. Gorging themselves.
Now Scooter hangs out all day with Banard. They only run and play very rarely.
So looking at it. It's just like humans you get sad, feed your feelings by feeding your mouth, and mope around changing your activity to inactivity.
I should help the dogs get in shape with activities and thus help my activity too.