I like the "it's complicated" theory, but it also a bit of a cop-out. Can we not have a meta-theory? Here are three candidates: 1) Climate change. Doesn't directly cause famines because people adapt; but the adaptation might be e.g to become wondering marauders. Thus war, destruction famine etc. 2) Technological change. Iron-working upset existing power balances. The political results were different in different plac…
Isn't a "systems collapse" the meta-theory you're looking for?