I can't say for certain, but I can probably make a pretty good guess.
During push-back the airplane is moved backward via a tug and towbar
attached to the nose gear. The nose gear must be free to turn to permit the
tug driver to steer the aircraft. The nose wheel steering system would tend
to resist his efforts if it was engaged.


I am not familiar with the 320, but all large aircraft have hydraulic
steering. Usually, the scissors are disconnected prior to towing, but
that company may elect not to for push back. In any case, it is most
likely a safety issue as bumping the tiller or rudder fine steering
while someone is trying to handle the tow bar or hook up the scissors
could make them loose some body parts. Some Boeings I am familiar
with have a switch above the nose strut so the tow person can turn off
the steering for the same reason.




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