Overpopulation

Warring factions (Old times)

Natural Disaster

Increased temperature

Neighbors that deplete recourses

Neighbors that are also collapsing

May also become a hostile nation

Trading partner is also a collapsing nation

Loss of a trading partner ruins economy

The Vikings

Depleted recources

Overuse of recourses

Rational Behavior

Irrational Behavior

Ignoring the issue/not noticing