There wasn't a collapse as most people think. There wasn't death and destruction left and right. The end of the Classic period did see an increase in conflict, that's true. Places like Aguateca had walls put up around their ceremonial center, Colha was a huge lithic production center with a mass grave, and La Blanca shows that access to the ceremonial center was restricted and blocked. While there was conflict and killing, against who is still uncertain, there was also a lot of population movement. Many people from the Lowland areas moved to the Highlands or closer to the coast. The exact reasons for the Lowland Abandonment are still uncertain, but several reasons have been put forth. Some areas experienced intense drought that they could not deal with in part due to the deforestation that took place around most of their settlements for cooking fuel and to make plaster. There also seems to be a political change in which the king is no longer seen as divine and rules with the aid of a council. There was also a marked increase in trade along the coast which may have motivated people to move. Warfare certainly played a part for some areas. Most likely it was a combination of all of these factors to various degrees.

The other poster linked to a Ted talk about Diamond's work on collapse. I didn't get a chance to reply to them, so I'll out these two articles here. They are rebuttals to Diamond's book on collapse and cover a number of different groups.

https://www.dropbox.com/s/4vxncpmincvkkgr/McAnany%26Yoffee2010.pdf?dl=0

https://www.dropbox.com/s/3oar99xpe91qnxu/Tainter2006.pdf?dl=0