Epidemics 

This theory initially sounds very feasible because of the destructive affect that diseases are capable of causing on a civilization.  The bubonic plague in Europe is a perfect example of this.  

According to the epidemic theory it was a combination of both malaria and yellow fever epidemics that forced the population of the Petan lowlands to abandon their cities and homes.  

The one major argument against this is that both of these epidemics are foreign to the Mayan culture. Malaria originated in Europe and the Yellow fever most likely has its roots in Africa.  This means that they would have had to be imported during the Spanish conquest.  This, of course, came after the main disintegration of their culture making it impossible for this to be the cause.