I've no idea what just triggered it in my campaign. As a game mechanic it needs some work both in terms of documentation and the politics implementation in general.

I'm not that distressed by it, it's the first interesting thing to happen in my campaign in 50 years, so honestly I welcome the diversion of having 10 stacks of uber Romans drop out of the sky like they teleported down from the enterprise right on top of Macedonia. And they are in fact wreaking havoc all over the place because they immediately proceeded to snag settlements and hit the ocean in every direction possible.

I also have to say I'm envious of their color scheme, could be I just like the change, but either way, the nice blue colors they are sporting look nice.

Anyway, no idea what triggered it, I was in the 30's. been managing everyone so they stayed even with me, never too low, never too high, just to see if it was avoidable, apparently it's not, which is a shame in and of itself, it should be possible to manage politics to avoid it, otherwise what's the point of politics, just ignore it and the event will randomly kick in anyway, or game it and cause it to trigger early to avoid the number of stacks I'm facing now (if you can figure out what causes it). Oh, imperium was in the last section, but by no means pegged.

Go figure. But hey, least I've got something to do for the next 5 or 10 years, haha...