Resource Depletion

In Diamond's book he presents five possible causes for societal collapse.  I believe that resource depletion/environmental causes is the major reason.  If you have a prosperous society that encounters environmental issues you will probably see several of the other causes that Diamond states.  Trade becomes important when you run out of resources because you are now dependent upon another countries commodities to survive, therefore boosting the importance of the trade argument.  If a surrounding country is involved in the same environmental issue then that company may also be likely to attack to gain access to what resources your country have have left as well.  Climate change can sometimes be related, but may simply just make matters worse.  Finally, when there is an environmental emergency or resource depletion, the government can help out or get in the way.  If you your country is composed of many cultures, a government may decide to give one culture preference to the remaining resources, which can in turn spark a wide variety of social issues, possibly even a civil war or genocide.

While this may not be the case for all societies, I give environmental reasons away for the largest contributor of societal collapse due to a close relationship between all possible reasons for such a collapse.  In a sense, environmental reasons can be a last straw that is pulled, triggering several other reactions.