Compulsive disorder in dogs

I sometimes observe that in our dogs particularly Pipoy, the hyperactive pug. But it is on rare occasions only. With biting the tail, most of our dogs did that when they were puppies but I never noticed that when they are already adults. Tisoy, our 8-year old spitz sometimes bites the hem of the pants when he wants to call our attention particularly when he wants to go out of the house. But definitely that's on purpose.