The nozzle is adjusted to manage the area of the aperture, this manages the mass flow of exhaust gases, maintaining the efficiency of the nozzle, and also managing internal engine pressures.
If nozzle pressures are too high, instead of flowing out of the nozzle, they can flow forward back into the combustion chamber, and in extreme cases out through the front of the engine - this is called an "un-start" and is...quite bad.
It is likely that the nozzle opens before the AB is lit as the nozzle is not directly controlled by the exhaust gases. If the nozzle were closed when the AB was lit, the sudden jump in exhaust pressure (caused by the increased mass flow without increasing the diameter of the nozzle) may well trigger an un-start.
tl:dr - open the door before you run through it.