I answered this question about a year ago here. Essentially, the word 'collapse' isn't particularly useful nor accurate in describing the Khmer civilisation. There were numerous factors and gradual processes which caused a 'change or transformation' of the Khmer from the 12th to 16th centuries, according to David Chandler. The rise of power in neighbouring kingdoms such as the proto-Siamese/Vietnamese states, as well as economic priorities shifting, perhaps even widespread religious conversion to Theravada Buddhism, had all been reasons regularly put forward in the past. More recently, thanks to technological advances, archaeologists have made the case that extended periods of drought and then powerful monsoon rains (read as climate change) was also a pivotal part reason for this transformation.