(The Destruction of Troy, Jan Brueghel the Elder)
This was the Bronze Age Collapse, the first Dark Age in recorded history, and the one everyone forgets about (when you think of Dark Age, do Egyptians battling the Sea Peoples come to mind? No, it's togas, Goths and vomitoriums) despite the fact that it was arguably more devastating than the end of the Roman Empire.
Well you add in populations already impoverished and war-weary from the great power conflicts that are now suddenly homeless. Disaster happens. Refugees streaming into neighboring territories worsen the situation even further.
It can happen very quickly: In such a strained environment, all that needed to occur was a trigger: the dice just had to come up wrong one year and everything began going to shit. Society's more fragile than it looks, and disconnected elites + taxed treasuries + disaster can easily send things spiraling into chaos.