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Your government will shift according to the decisions you make in game.

There's a button you can click in the Government panel to view those decisions (and the strength and direction they have shifted your government in (+1, +2, +3, +5, etc.)).

Likely you've built a lot of a particular type of station, comported your foreign relations in a non-militaristic way, and balanced your economy between social and mercantile interests (if I'm recalling correctly).
 
Hmm... it seems that building lots of colonizers moves me toward cooperative government, well I've got some money to burn might as well build bunch of colonizers only to scrap them later. Bit silly way to get government you want.