As we concluded this 15% Unrest is tied to the 3% probability of rebel spawning; but today I observed something different, and would like to share with everyone who is reading this thread.

Playing as England, at turn 45, still with London as the capital. Nottingham has a Public order of 140%, no "Distant to Capital", no "Infamous 15% Unrest", no "Religious unrest", it only has some "squalor", but a rebel army still pops out. (The city has 4 groups of Spear Militia garrisonned in it, nothing else)

So I suspect, besides the obvious 15% Unrest, there is still other factor(s) causing rebels to spawn.