2. It becomes clear that the society is inherently opposed to the PC's goal.
3. The PC's destroy the society.
I've never known why this happens. No-one ever plans any part of it. I make up a society, the PC's make up a goal, and the way our minds collide in play somehow naturally disintegrates into this pre-ordained pattern, to the constant surprise of everyone involved.
Here's an example: One of my players had their heart set on creating a democracy. After impersonating the ruler, he gathered all the nobles in town together and told them that they were all going to be demoted. He was actually bewildered and heartbroken when this didn't work. "My utopia!" he cried, as the party thief slaughtered noblemen.
Here's my attempt to explain it in detail.
1. I have a basic idea of how the session will run - a pre-made dungeon, encounter, something like that.
2. The PC's either ignore my pre-made stuff, or blow through it in record time and keep playing.
3. Forced into the delirious Improv Zone, I make up a Society on the spot. Even if I've got one that's been made up before we started, the PC's will force me into spinning up the details of some part I hadn't thought about. ("What do they do with their dead?" and then "Could we ride the corpse pillar down the death-pit?")
Often the details will be spun from the PC's themselves - they'll make a joke or something, and I'll go "Sure, that's how it works".
4. The PC's case the joint. As I make up the society, they research it, develop a goal or a way to achieve that goal, and start putting plans in action.
5. The "Gravity" of my campaign pulls towards escalation. It is very hard to calm things down, and very easy to make things worse. Thus, even if the PC's are in a calm situation, they soon find themselves sliding towards destruction and anarchy. (This isn't a conscious decision on my part - just the natural effect of what I'm interested in, probably.) Naturally, they eventually end up fleeing the ruins of society.
6. Because of this society-destroying style of play, solving a problem naturally creates a dozen more. The PC's have a bizarre amount of agency on the world - far more than anyone could have in real life. Everything they go through is warped or devastated until the game world is entirely defined by PC-created problems, and I don't have to make any plot hooks on my own at all. The players have made their own world to play in.
It kind of emulates Jack Vance's Cugel's Saga. The society is a trap for the protagonist, and the last page is the protagonist fleeing as the society is destroyed behind him. It's a strange, wonderful thing, and I've loved running it. The last session has finished, though: The players have achieved a picaresque kind of closure, with everyone dead and the last city they tried to save still destroyed.
I'm exploring a more classic kind of dungeon crawling, now, trying to fill in my weaknesses: I never understood XP, Treasure, Dungeons. I had the players level up whenever they wanted. When they reached the treasure vault of the thief king and asked what was in it, I replied - "Uh ... what do you want?" I've never had a clear understanding of giving consistently upgrading challenges and abilities in an arena of deadly exploration and combat, and I'm going to give that a go.