What caused this large empire to eventually fall?

Invasion by Barbarians

Explanation:

The most straightforward theory for Western Rome's collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire's borders

i also think he is correct. i believe they came to washington to protest their rights.