The timing is suspiciously close to the destruction of classical civilization by the Muslims.
Much of coastal cities were destroyed/abandoned because of muslim pirates at about this time.
A Muslim raid against Rome itself occurred in 846 A.D.
Probably started as an infrastructure problem, the maintenance of the water and sewer system in imperial times made those large towns viable and kept them that way. The population scattered into smaller clumps, and the architecture got used as a quarry for later structures.