Climate change and settlement abandonment
Mesa Verde is a large sandstone plateau with deeply incised canyons. The buildings in the Cliff Palace date from around 1190 A.D. when the Ancestral Puebloans moved down from the plateau.
Why they chose this spot in the landscape is not clear; it is not the most easily defensible site, and the climate makes shelter less important. Droughts were beginning to affect the area, reducing the amount of cultivable land, and so one possibility is that by moving the settlement from the plateau they freed up more land for food production. This idea is supported by the fact that barely a century later, as the droughts increased and firewood and the number of game animals declined, the Puebloans were forced to move again, and so abandoned this settlement .
Mesa Verde National Park (U.S. National Park Service)
Kohler, T. A. (2013) Leaving Mesa Verde: Peril and Change in the Thirteenth-Century Southwest. University of Arizona Press. View on Google Books