The rise of the Eastern Empire: As Rome needed more medical help, more soldiers, and was squandering on the battlefield the Eastern Empire was getting a ton of money. This allowed them to get a good army, and help invade Rome.
Over expansion and Since they had such a large area now they were struggling to spread the soldiers out to defend the area from outside attackers, and rebellions. It was hard for them to fund, or to get a large army when it was such a large area for it to be around plus they need to buy the supplies, food, weapons it was incredibly expensive.
There was also a large corruption in the government of Rome. In a short time of less than a hundred years over 20 people took the throne. Most of this was because of the underling killing the predecessor, and becoming the new emperor, that's corruption for you.
Economic troubles and over reliance on slave labor: The inside of Rome was collapsing too because there was a major economic problems. From the rich, and the poor was a huge money gap. The number of slaves was decreasing which was causing them to not have much agricultural workers. That put a huge drop into the amount of food they had.
Invasions by Barbarian tribes: They were strong but we're being attacked by groups of barbarians. From outside the borders kept getting attacked from pacific places, and it weakens the armies of Rome. From this some places like Italy fell, and Rome would never claim them again.
And probably one of the strongest causes of the downfall was there soldiers. One they couldn't recruit enough soldiers because of the Mass area, and people didn't want to do it. There soldiers were dropping like flies from the rebellions, barbarians, and outer forces. I'm guessing this made it incredibly hard for them to keep an army.