From wiki, it seams every reason has a piece in the collapse. Here is what I comprehend of it.
1) Begining with poor weather that forces people to farm on marginal land
2) Farming on marginal land which destroy the fagile ecology
3) A collapsed ecology generate less food. Hence famine
4) Famine causes war and disease
5) War makes trade route to be cut off
6) Less trade means less food and even more war. Creating even more famine
7) Which in turns forces people to farm on marginal land again, and creating more havoc on the ecology, etc...