My pet hypothesis is : That trade networks, in times of collapse- become sort of superspreader networks of downfall. Think about that city state who runs out of food by the sea! It still has all the trading vessels- whats more logic then to go - and take over somebody elses city and ships! Piracy of the damned! Stealing the food from the starving, just to give there families one more day! Following the coast- until you run out of city- and the civilization is gone!It should also not affect the country interior cities - who then would murder the upstart pirates who took over the old capital near the sea..

The Ottoman embargo of Western Europe leading to the collapse of the Aztecs supports your hypothesis. The other sea people.

That's just the Sea Peoples hypothesis