How far did push factors lead to the decline of great zimbabwe?

The very factors that led to the rise of Great Zimbambwe are the same reasons that led to the its fall: 
a.The rise Exhaustion of soils due to continuous farming. The plateau was exhausted by overpopulation of people and animals.
b.Scarcity of important resources like firewood and pasture and drought.c.Civil war from poor leadership decision.

Acase involving the laws of a single state.