Two thoughts come immediately to mind.
1. In the Middle Ages, the length of the toes of a man's shoes showed his level of wealth. A guy with really long toes on his shoes obviously didn't have to walk anywhere much. The got so long they were being tied to the knees, and eventually even the belt.
2. AJ Foyt once said that if a car went around the track two miles per hour faster than everyone else with a Christmas tree tied to its tail, tomorrow all the other cars would have Christmas trees tied to their tail. Clearly this applies to yellow tail pipes, too.