If you switch the engine off immediately on stopping, after a long ascent, after constant speed on the motorway or after driving fast then a pump will continue to circulate coolant through the extremely hot turbo as it cools down and the heat may be sufficient for the thermo switch to switch on the fan. It is best to idle the engine for a minute in these circumstances before switching off as that also circulates the engine oil through the turbo making it less likely to carbonise and block the turbo oil feed and oil return pipes.