It has spent quite some time at 100% as I have done A LOT of questing.(I'm fuzzy on this, so take it with a grain of salt.) I believe there is a certain amount of time that must pass in game before a city will upgrade. I don't know if it's a setting that can be disabled, nor when exactly that clock starts ticking, nor how long it is. I could also be completely wrong about this, so hopefully someone more knowledgeable will pop in and set me straight.