Conhistory/Peoples/Push and Pull
The Push and Pull factors in Conworlding are things that 'Push' people away from an area of land and 'pull' them towards a new one. For example if an area of land was to change from lush savannah to inhospitable desert much like the Sahara did then that would pull large amounts of people away from that land to a more fertile and forgiving environment much like the Nile Valley in the Sahara. As you may be able to guess this is how the Egyptian Civilization began to form, from migrating Saharan peoples who found the land in the Nile Valley more habitable than the Sahara which at that period in time was drying up very fast.
The Push and Pull factors often cause people to migrate and have caused such incidents as the Founding of Egypt, Mongol Invasions, Colonization of the Americas and the Polynesian Migration across the Pacific. The Push and Pull affect does have a real and large presence in the History of Earth.