There are more than 3 different explanations about the collapse, and all of them are theories: we don't know for sure, and we might never find out with certainty.
Some of the theories proposed concern a foreign invasion, especifically by the Totlecs. Another suggesion is an epidemic, which could explain the rapid decline in the numbers of population. Yet another theory considers droughts as the main reason and in fact there is also scientific evidence for such droughts.