I would put climate change in the first place. For exemple mass deforestation in the Anckor plain due to the increasing urban system of the Khmer empire made the terrain more unstable and the canals and water bassins (barays) to be more frequently blocked by mud. As the phenomenon increased, the cost became unbearable. At the end of the day the (conglomerate of) cities that covered the Anckor plain (Anckor Wat is only one of the many temples) were abandonned and the Khmer ceased to be a regional superpower. Of course, as in the Khmer case, those climate changes could be initiated by men, mostly due to an overexploitation of the available ressources... I don't know the mayan case very well but it has striking similarities.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, you may PM /u/caffarelli to have your question considered for an upcoming Tuesday Trivia thread.