“I’d add a loss of food supply,” states the next person. “Cities typically import most of their food. If something cuts off that supply, people will leave and go where there is food available.” This problem is unlikely to affect America anytime soon.
The expansionism of late 19th and early 20th-century America meant that towns would disappear just as quickly as they appeared sometimes. “Happened all the time in the old West,” asserts the next commenter.
“After the Romans left Britain and the Anglo-Saxons started invading,” explains a history fan, “the left-behind Romanized Britons abandoned many of their cities and fled west to get away from the invaders.”