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..