As I understand the history of this (in very broad terms). a few years back Eurocontrol was setting some rules for tx'der ops on the ground to support A-SMGCS use. For whatever reason, they chose 1000 for the mode A on the ground. parked up etc. and publicised this widely. When it went to ICAO they decided that 2000 was the code that should be used.
Hence the change at those few airports that have had A-SMGCS for a while and previously were promoting Eurocontrol's rules.
Off the top of my head I think the ICAO reference is Doc 8168 and maybe Doc 7030 too.