[Ocean]

I have read that it was because when the Rottweiler (which translates to "Red Tile") was originally bred/used as a working dog in the town of Rottweil, Germany, they were known as the "butcher's dog" because they pulled the meat wagon for the town butchers. Their tail was originally docked because it got caught in the spokes of the cart otherwise.

Anyhow, this is what I read.