My hypothesis is what I call "the ISIS factor". Remember when Bush dismissed the Iraqi military to unemployment while they were being ethnically cleansed? The ex-military guys joined ISIS and suddenly ISIS was a fighting force that took the world's most powerful militaries months to put down.

Now, imagine the drought and disturbances prior to the end of the bronze age. Refugees started, but they were easily put down/enslaved/whatever until the military guys of crumbling nations became refugees themselves and said, "hell with begging, let's switch to taking". They became a desperate but well trained fighting force. As former empires crumbled (refugees disrupted the trade that brought copper and tin together -- the Bronze age very literally ended) there was no "best military" left to fight these now, now militarized roving groups.