Heater/warm air line would be my guess too...An attempt to get the cat hotter faster so it works at peak efficiency.
Another guess, since it is going toward the back of the vehicle would be some type of EVAP system for the gas tank? Gasoline fumes being directed to the cat to be cleaned up prior to being sent to the atmosphere?
Maybe pull it out of the cat and see if there's hot air blowing out of it?