If the Aristocratic Coup Disaster ticks to 100%, and you're not a Noble Republic, your government is coup'd into becoming a Noble Republic, with your current ruler as the head of state. It's an unavoidable consequence of letting the Disaster tick to 100%. Think of it as, well, a bloodless coup, not a rebellion.
When a faction reaches 80% influence or above (though the tooltip only states "above 80%"

), the Disaster starts ticking at 1% per month, with incrementally higher ticks if their influence rises higher. At 100% influence, the Disaster ticks for 5% per month, which means the Coup will happen quite fast unless you do something drastic, like revoke a lot of the relevant estate's provinces.
As far as I'm aware, you won't get any revolts or rebels from an estate having 80%+ influence, but once that Disaster ticks to 100% and activates, you'll be hit with some nasty modifiers and forced to change your government (with the Aristocratic / Plutocratic Coup but NOT with the Dominance of the Clergy disaster). Atleast with the aristocrats; full disclosure, I haven't tested the other estate coups.