mine does it too if you shut it off during a regen and then start it back, only if its kinda cool outside though. i think whats happening is its going through regen then when you kill the motor whats burning inside the dpf smolders out. then when you crank it it doesn't go back into regen so the normal exhaust flow blows the smoldering funk out the tail pipe. thats my best guess. next week that piece of junk dpf is coming off my ride haha