Answer:
Invasions from Barbarian tribes, The rising of Constantinople, Overexpansion, Government Corruption and Instability, Economic Troubles, Slave reliance for money, and Christainity rising.
The American Revolution led to the Loyalists (Tories, to you Americans) flooding north and helping to populate the Maritime Provinces and what became the Eastern Townships of Quebec. They also, pretty much on their own, created what's now Ontario.
