Does it depend on the airport as well ? I did a flight from KPHX- Loading had only 1 pallet. When I landed at KPDX it unloaded
No, only by the estimate passenger number, which depends either from the payload, or can be manually set if you disable the Estimate Passenger number in the settings.
That option is useful to prevent confusion caused by the airplane itself unloading its payload before you call GSX.