Society slowly turned into an oligarchy where a handful of families owned the majority of wealth. And the same time depletion of natural resources, mostly forests, lead to systemic scarcity of fuel (wood). Increased social complexity due to the size of the empire. On top of that, ongoing wars with barbarian and Ira... I mean Persian forces.

Source: https://en.wikipedia.org/wiki/Historiography_of_the_fall_of_...