I think they are due to more specific events. The first two I am sure: every time I found my second city, I earn my first palace upgrade. Might have something to do with territory controlled, as the second city gives me more, but not culture or power, because they always expand the palace regardless of what turn I'm in, which could mean more or less culture or power. The second comes soon, the first time one of my cities defends and wins an attack, either by another civ or a barbarian, doesn't matter what turn it is/how powerful I was/how many cities I had. I've also guessed another two: when you have all 8 luxuries, and the first time you eliminate another civ. I'm currently playing a game and I'm close to acquire my last luxury, I'll pay more attention. I forgot to check about the first civ I eliminated though. Anybody cares to test?