Very likely a mass kill off from a lahar (volcanic mudslide). The Basin of Mexico (Mexico City is built over an unstable lake bed) is surrounded by volcanoes.
Another possibility. Myself, I'm inclined to believe they met their fate the same way most of the other extinct megafauna did in northern and western hemispheres. Fact is, there have been indications from other parts of the world where tsunami type detritus was mingled with volcanic ash, mangled trees and the like -- along with sand and gravel -- from somewhere.
The end of the Pleistocene was not a pretty picture. Our most recent and probably least studied extinction event on the planet. Too close for comfort maybe.