I'm taking a class in basic archeology right now and I am throughly disgusted with all the theories about why archaeological sites were abandoned.
I can you why they were abandoned. There was a problem. Why problem? Essentially there are two reasons cities are abandoned, economics and disaster.
Economics means that previously available resources that provided for support of the city have declined or failed. Abandoned "boom" towns are created when, silver mines, coal mines, niter mines running out or the market for the resource disappears.
A disaster means earthquake, plague, volcanic eruption, anything which makes people feel as if they can no longer live in the area.
Political problems lead to revolutions, not abandonment. Detroit is
not abandoned as much as some white supremacists like to pretend it is.
Is Detroit thriving? No, but it isn't dead either.
figure out why people abandon cities we need to look at why people
abandon cities. Look at the ghost towns, the abandoned cities of today
and we can learn why cities are abandoned. Once we understand the
reasons why people abandon cities today we can systematically eliminate
the various possibilities until only one remains.
There is another kind of disaster that I think probably caused the desertion of some major cities of the past, although, it would be difficult to prove. Population pressure causes pollution. I believe that the water supply of some ancient cities became polluted and caused what seemed to be a plague on the city. People left because they could no longer stay. Overtime the ecology cleaned up the problems.
Now that the basic inductive logic is over we can address some speculations.:-)
I'm not sure it would have taken very many people to die for people to leave. How many deaths does it take for a group to migrate?
Rome was all but abandoned because the water supply was destroyed. If population pressure polluted the water supply it could have driven people out of cities.
Population pressure also creates the potential for an actual plague.
Recently archaeologists have begun blaming a volcano for the abandoning of the Mayan population centers. I think it is possible that smoke or volcanic ash was the source of pollution that made required resources unavailable.