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).