Isn't this just the natural progression of corruption in a nation?
As for why it wouldn't immediately explode, well, that would imply the subjects of the nation are willing to stand up and do something about it. If you give yourself a lazy populace just as corrupt as the government that runs it, I think it would be stable enough to last a while.
Imagine a society where everyone is unhappy, but everyone decides they are somehow able to take the role of the observer, and say that their quality of living is someone else's problem. Leaders are constantly replaced by people even more incompetent. At some point it's going to explode, sure, but not immediately, because that implies someone is willing to actually do something about the situation.