One theory of the late Bronze Age collapse I've heard is that great numbers of peasant types ran off to avoid governmental pressures and became nomads and raiders that eventually attacked and took down the large states.

I'm not sure how much weight I would put into it, but one story I've heard seems pretty solid: The Philistines (of biblical fame) were Mycenaean Greeks who attempted to attack and settle in Egypt, and who were in turn resettled on or near the Egyptian border with the Levant.