Tail docking got started in the OLDEN days...when the people who made the decisions about whether or not a dog was good quality would come out, look the dog over and if the dog passed, they docked the tail to keep them separate from the ones that didn't pass.
It reminds me of a joke I heard once, a young bride was preparing a roast to fix for dinner...she cut the ends off of the roast before she placed it in the pan. When her husband asked why she did that, she said, "because my mother always did." so, one day as they were visiting the mother, who was fixing roast for dinner...he watched as she cut off the ends...he asked why...the mother said, "because my mother always did" and so, the first chance the husband had, he asked the grandmother..."why do you cut off the ends of the roast before you put it in the pan?" The grandmother replied "I had to, my pan was too small!"
Tail docking, 101.