The first possibility is that there was just a giant earthquake, and not only would that kill lots of people, but that could have literally moved the river. And if you move the source of all life, you're no longer the master of your own domain. So they must have had to pick up their stuff and kind of move along the road there. You also have the possibility of invasion. To the north, you have the Caucasus region. There were a lot of nomadic tribes and warriors. They could have came through the different passes and just invaded.