2) Over Expansion Over expansion is my second choice for over expansion because if an empire gets too big, it gets really hard to govern, and since they didn't have our technology today it must've been very hard to communicate.
1) Political Instability Rome's Political Instability is a reason it fell because if an empire does not have a strong government with a good emperor it will be an unstable empire. For a stable empire you need a clear line of succession first and fore most.