My two cents. The question isn't how did it fall, but how did it last so long. The fall of Rome was inevitable. A city could not continue to govern such a large area. The growth of population in the east, north and west made it impossible for Rome to control these areas.
For a couple of humdred years, the Romans would win a battle and then hire the vanquished army to serve as the Roman army in that territory. This practice along with splitting the empire into thirds, made it inevitable that Rome would fall and that the seat of power would be elsewhere.
The Brits did the same thing -- by the start of the 1800s they were the dominant power. Then, they started to think themselves a superior race and had become increasingly effeminate. They would have been knocked out by Germany if we hadn't gone to save their sorry behinds twice.