Maybe the external factor is the final kick, knocking over a rotting house of cards, but the cause is almost always division, internal conflict, or unsustainable growth.

Rome was a place where power consolidated, but the various eras might as well have been different empires.

The system collapsed, dissolved, reconstituted, and expanded several times during the 1400 years it existed.

Same with China. 5000 years of history emerging from Beijing, but each dynasty was distinct.

Even the US has reinvented itself several times over by now. Antebellum America might as well have been a different country. New Deal America was radically different from it's Coolidge Era predecessor. Reagan's America became it's own thing in turn. Trumpian America is a new thing, not an end point.