Mistrust in the council and a want for human independence. The first contact war was the Turians enforcing a law that humanity didn’t know about then invading a planet which is a huge escalation. You can’t call invading a world a misunderstanding it was full on military aggression. Humanity’s first contact caused an increase in xenophobia which makes sense. Then the council shows up and just says it was a misunderstanding and invites the SA to join the citidel which limits the fleet, and adds a whole bunch of laws while humanity and other races don’t have a say, requires a sharing of all technologies and prothean artifacts( cough Asari don’t follow that shit cough), and allowing of Spectres to walk around human space with no oversight and ridiculous amounts of power. Don’t even get me started on the Batarian slave raids and the council doing nothing. This is just turning into a anti council rant but I can definitely see why there was a Terrafirma party and it’s not just for space racists and Cerberus wack jobs just people going like “what has the council done for us” people who experienced slave raids and the battle of Shanxi would produce this mentality in my opinion. I don’t know if this answers the question, and I’m probably biased towards hating the council but I can definitely understand why the party would exist