One has to suspect a thermostat problem, and it's easy & cheap to replace to check.
If the engine is running hot, the thermostat may not be opening properly, only allowing a small amount of hot water to the radiator, which will be cooled quickly by the passing air. When you stop, and there is no passing air, the hotter than normal coolant will flow down the radiator until it reaches the thermoswitch, and the cooling fan will then turn on. Thus the time from stopping until the fan comes on is shorter than normal, because you do not need to wait for the whole engine and coolant to get hotter.
Hope this makes sense.
P.S. There is much debate on this topic in the thread I referenced above, as well as details of the model year changes to the cooling system controls.