[tonythetarga]

Mold spores are everywhere and, no doubt, in your ductwork. While using the air, the ducts get cold. When you shut off the car, the ducts warm up and may form some condensation inside the ducts and just enough moisture to urge the mold to grow and emit more spores. Fast forward as you say, and by turning on the airflow you sent the spores airborne within the car and up your nose.
This happens in almost all cars and many new cars have HEPA filters in the cabin system to trap them before they become airborne.
One thing to consider is to turn off the air for the last 5 minutes of your drive and before you turn off the car. Turn the temp all the way warm and leave the blower on to circulate warm ambient air. It will warm up the duct and not allow any condensate to form when you shut it off.