I know this because it just happened to me, in my favor. Some of you probably already knew that because the game tells you it'll happen once a civ reaches -20 unhappiness, but for anyone who hasn't gotten into the game yet, here's another neat thing to get excited about. Spoiler : I'm playing on Emperor, going for massive culture to try out the new mechanics (very fun btw), and my influence is everywhere. By Industrial, I'm influential or dominant with two other Civs, so when I pick an ideology, the rest of the world listens. I've picked Freedom, but Shaka decides to be an Autocrat; because my culture is very popular with his people, it caused their citizens to dissent. Once Brazil, another high culture civ, went Order, the pressure to change ideologies was just too great for Shaka to handle, and his citizens went into full on revolt, leading to a city defecting to my nation. Now, the culture flip happened because a lot of very specific circumstances lined up at once: my culture was quite embedded into his (not Influential yet, but Popular), which caused Ideology pressure in my favor (citizens like the ideology of the culture they like best). I was not Influential yet, so this, on its own, was only enough to get his people to make some noise (Civil Dissent), but once another culture they really liked (Brazil) went something that also wasn't Autocracy, they started to hate their government, and Shaka's Ideology Approval dropped all the way into Revolutionary Wave, smacking him with a whopping -80 Unhappiness. He stayed this way for several turns, and since my culture (and thus, preferred ideology) was at the top of their list, a city adjacent to my borders culture flipped, and joined my country. This was probably the best way for Culture Flips to make a return. I'm so impressed at seeing it, and was pleasantly surprised to discover that it was, indeed, back. It took me totally by surprise, because all of a sudden I get a popup asking me if I wanted to annex/puppet/raze a city, when I wasn't at war, and then I realized what must have happened. The city I happened to get was also one that I had my eyes on for a while, so now I don't have to DOW him for it.