One of Rome's most important jobs was farming. Without the land for soil, Rome was now put in a economic downfall. I chose agriculture for the third reason because when all the land wore out and the empire couldn't feed everyone, this caused unrest throughout the Empire which led to distrust of the empire.
The Roman Empire became to big for someone to control effectively. They now had all this land to make sure wasn't being attacked which meant their spending were going towards fighting off invaders and paying soldiers. Because of this the empire had less funds for maintaining infrastructure. Soon government officials became greedy destroying the sense of citizenship that had previously been in Rome. This erratic behavior rubbed of on the other citizens and many Romans no longer felt to have a duty to the empire and chose to get rich. I chose over expansion for the second reason because if you can't control the citizens or the land, your empire is on eggshells.
To create a stable empire, you need a stable government. Rome's government during this time of downfall was all over the place. After the last of the 5 good empires passed, the next 22 empires died a violent death causing the citizens of Rome to question their trust in the political leadership. If your citizens don't trust your government, chances are your government won't succeed. I chose political instability to be the main reason Rome fell because without a string government, the trust the citizens have in you decline and everything else falls apart.