A large populace held in check by a small but powerful force is quite a common situation in our universe. And we know the major conditions wherein this large populace may turn upon its keepers --
When they find a leader. This is the most volatile threat to the powerful; they must retain control of leaders.
When the populace recognizes its chains. Keep the populace blind and unquestioning.
When the populace perceives a hope of escape from bondage. They must never even believe that escape is possible!