Great question!

The author unfortunately didn't have a clear idea of all the causes, he was just able to recognize something fundamentally off about it all, and used Mayan civilization as a prime example, claiming that Mayans simply walked back into the jungle and never engaged in civilization building again (his claim, not mine.)

It confused me at the time of reading, and it deserves to be revisited again by me, armed with insight from Softwar. Looking forward to it!