The number one contribution to the Roman Fall was the plague, it was responsible for the death of a lot of Romans and others during the Roman era. The Roman Empire was severely weakened after the plague took place.
The corrupted ness of the military helped contribute to the downfall of the roman government mainly because not only did it make the economy in Rome horrible but it also weakened Rome's defense and offense.
The Romans were so good at their empire they managed to expand greatly over the years. However this also had a negative affect as well as a positive one. Having too many land and citizens makes an empire harder to run therefore helping the Roman Empire fall.