[bwana50]

Hmm, I'll take a stab at it.

Mass flow = density*area*velocity. Density is constant and mass flow should be constant, so as area decreases (door closing), the velocity through the door increases. There is probably a point where velocity cannot increase because the fan can't move the air that fast due to various ineffeciencies. (Theoretically, if the fan were perfectly efficient and the house well-sealed, the fan could move ALL the air out and create a vacuum.) Then mass flow would decrease and the fan stalls. It's still moving air, but not like it should. Thus the apparent decrease in velocity at the door.