Sacred Haven of Debauchery

No, they are not happy that I joined the federation. And they call themselves imperialists. In that case, it's not surprising that they are unhappy. Well, in short, I messed up.
The traditionalists are most unhappy because of the robots and robotization.

And this is what the militarists are. As far as I remember, only two factions are against federations: isolationists (a mix of pacifists and xenophobes) and militarists.

Added 4 minutes later

GLORY TO THE ROBOTS, KILL ALL HUMANS

In Stellaris, the opposite happens. It's not that you suddenly start attacking organics; it's that organics start treating you badly (unless you are materialists yourself). Especially spiritualists.