“fall of civilizations” podcast episode 4 (https://fallofcivilizationspodcast.com/2019/03/26/episode-4-...) has an interesting, and alternative, take on this but to distill it, like so many other episodes in the series:

climate change, man-made or otherwise, and the need to adapt pushed humans beyond the conventional borders of their tribes/clans. however, the precipitating event on the move from the mainland was a lot of murder.