2008 CX9 have defective heater relays for front and rear blowers. When you switch off the engine, sometimes the blowers turn on even if every heater control is off. But when you turn on the igniton, the blowers turn off. Until you turn off the ignition again, then the blower motors turn on again. But like you said, they turn off after a while. Change the heater/blower motor relays.
