Multiple causes, and it wasn't a complete collapse all at once. Shifting climate patterns combined with their habit of coating everything in lime plaster which required burning massive amounts of trees also contributed to it. Eventually the biggest cities couldn't support the populations that they once had. The same thing has happened many times throughout history. Angkor Wat, the Indus Valley Civilization, the collapse of Egypt's old dynasty, the bronze age collapse. If people can't feed themselves and their families they move on.
