well formed Autocrats are harder to break, mainly due to Autocrats needing more happiness than others for war purposes.
Between the happiness from castles+ (+1 here) and the happiness from Barracks+ (+2 each here), they can maintain more local happiness than others. If they had access to a tonne of unique luxuries and built the Prora (mass happiness for SPs) then they can withstand some problems. Check out which SPs they had and if they had mercantile CS allies or other ways to get happiness (such as wonders).
edit: and Deity AIs still get a 25% reduction in unhappiness (from Pop I think).