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[wof]

Why do some pilots set transponder code 2000 (usually after landing) in some aircrafts like Boeing 737? Any ideas?

[Intruder]

It is an innocuous code that will not affect anyone if inadvertently activated.

[Spitoon]

In many ICAO Regions A2000 is used for aircraft which have not been assigned a transponder code. As Intruder says, it's a 'safe' code to leave set.

[fantom]

It is what you do to avoid the next take-off transmitting the last flight's code (that has not been re-set).

[Too Few Stripes]

It's nothing to do with aircraft type but everything to do with the country. It is a notified ICAO difference in the UK for example.

[Jumbo Driver]

In UK, it is a requirement of the AIP under ENR 1.6.2 - SSR OPERATING PROCEDURES, para. 2.1 (e), which reads:

2.1 Some Mode A codes are reserved internationally for special purposes and should be selected as follows:

(e) Code 2000. When entering United Kingdom airspace from an adjacent region where the operation of transponders has not been required; or by Mode S transponder equipped aircraft on the aerodrome surface when under tow, or parked and prior to selecting OFF or STDBY - unless otherwise instructed by ATC.

JD

[framer]

In Ausi it's 3000.
Like has been said, if you forget to load your assigned code, it will already be transmitting the code for a/c who haven't been assigned a code