" It has been reported that operators are failing to adhere to requirements for Mode S transponder equipped aircraft to select Mode A Code 2000 when under tow or parked (prior to selecting OFF or STDBY). As a result, ground systems are incorrectly pairing Mode S transmissions, which is resulting in unnecessary delays in procedures and potentially overloading critical ground systems."
https://www.caa.co.uk/docs/33/SafetyNotice201107.pdf