Happened to me twice in my last completed game, and once in a game before that.
The first game I had about 140 cities, was on a domination campaign, and at the time was not the agressor. I had universal suffering. Most cities were in WLTK, there was no disorder. My army was in home territory--though that was conquered territory.
I think the trigger was the proportion of home cities--ie cities that had always been Persian--to the number of conquered citiees, and there fore the 20 native cities were supporting 100 non-producers. war was about 25 turns old.
Last geme, first time I was not the aggressor the first time, and war had only been going about 15 turns. No disorder anywhere, and about half the cities were native cities. No clue.
2nd time, I was aggressor, but i know the war was less than 20 turns. Had universal suffrage, war was short, no disorder, had 150 cities, 30 or so my own home cities.
Now if I know I expect war, I go back to republic.