Could they have been driven out by nomadic tribes, such as Utes or Navajos? There is no direct evidence that either group, or any other like them, was in the area that early. There is mounting evidence, however, that the Numic-speaking peoples, of whom the Utes and Paiutes are part, had spread northwestward out of southwestern Nevada and were in contact with the Pueblo-like peoples of western Utah by A.D. 1200. It is certainly possible that they were in San Juan County shortly after that. Ute and Paiute sites are very difficult to distinguish from Anasazi campsites, and we may not be recognizing them. Navajos were in northwestern New Mexico by 1500, but we do not know where they were before that. Perhaps the answer to the Anasazis' departure from Utah lies in a combination of the bad-climate and the arriving-nomads theories.
See: J. Richard Ambler and Marc Gaede, The Anasazi (1977); and Linda S. Cordell, Prehistory of the Southwest (1984).