‘And Lot also, who went with Abram, had flocks and herds and tents. And the land was not able to bear them that they might dwell together, for their substance was so great that they could not dwell together.'
The riches gathered in Egypt have altered the situation. There is no longer room for both sub-tribes to stay together. This begins to cause friction between the two sub-tribes. The land is just not sufficient. They must seek wider pastures.