The civilizations all eventually catch the disease of Liberalism. This leads to de-evolution and eventually death.
Semantics- what are liberals-or ‘progressives’- but the stormtroopers of bureaucracy? More bureaucracy is all they ever offer.
And I’m not kidding. I see no evidence in history of anything that destroys civilizations like their own bureaucracies.
Much sci-fi, notably Asimov’s ‘Foundation’ series, incorporate this idea.