Triggering points were different too and can't exactly be separated so smoothly from geography (there was an urban rural split too in where the violence occurred), here's a tldr; The Trouble was a question of rights of a nation to self-determination, Years of Lead was more political-social-economic determination.

I figure the US will be a bit of both in there are states or regions that want to secede no longer feeling they belong to the country because of religion and political reality (The Troubles, though not quite since the US isn't a true nation), and there are fascists that are out killing leftists/whoever they don't like, which will eventually lead to retaliation (like Years of Lead) since people don't exactly sit there and die when they can see an obvious causer of death and this will radicalize some (a lot of it was paid opportunist faction sadly), unfortunately most will become ultras or ideologically confused. In Years of Lead there was a monarchist-funded Maoist Hitlerite group, in our version Lyndon Larouche will getting his paid patsocs to join up with Patriot Front or something.