Unrest seems to be one of the better understood variables in the game.

- Each city has a base unrest of 30, 15 or 0 (see unrest thread linked by zhuge for city list)
- After a takeover, unrest increases by 30 and then declines 5 per turn back to the base level

If you have a city with unrest that doesn't fit this model, odds are it's a spy or your governor's VnV. And it's fairly easy to check to see if your governor have a vice related to unrest.