You didn't read the links I provided.
Found this little tid-bit that mentions a decline in specifically 540AD.
"Tikal was one of the winners in the new order, making a strategic alliance with Teotihuacan, the major power in the Mexican highlands, and rising to become the dominant regional power. Then, with Teotihuacan's decline around 540AD, Tikal entered a long hiatus, which only ended in 692 with the accession of ruler Ah Cacau Caan Chac.
It was during Ah Cacau's reign and that of his successors that Tikal reached its peak of building activity, artistic achievement and population. Numbers rose from an estimated 45,000 over the surrounding 50 square miles to as many as 70,000. Then, in the early 9th Century, Tikal entered its final decline. Building stopped and the quality of its ceramics coarsened. The population collapsed to a mere 3,000 to 4,000 people who looted the tombs of former rulers and dumped their refuse in dark corners of abandoned palaces. By 1000 AD, Tikal was left entirely to the encroaching forest. Why?