Have never seen anyone specifically address that question. In the absence of better information would have to speculate that it was "for the usual reasons".
But Dr. Iben Brownings thesis that changes in climate due to unreported volcanic activity was one of the biggest reasons for migrations makes sense to me, in both ancient and modern history. (The Little Ice Ages in Europe as a prime example.)
That's what I was looking for, being a catastrophist and such. Hee, hee. Give me an exact date and I'll find something. lol.