I don't know how it compares to more recent scholarship, but my impression from reading Braudel is that in the ancient world there was a sort of wave effect. People would migrate out of one area - pushed famine, overpopulation, etc. - and would then push another nomadic group to move on, who in turn pushes another group. Sooner or later this movement would hit more settled lands, but the original cause of the movement could be obscure.

Nomadic people also tended to have an important military advantage in their horses and riding skills. In the early ancient world this technology was particularly decisive, but even in the Mediaeval world, the mobility of groups from the steppes was very powerful.