Is that true? Is that when the Vikings started raiding?
Technically yes. The Hundred Years War between England and France also ended when Constantinople fell to the Turks. but in neither case were the eventrs related.
> in neither case were the eventrs related
Not necessarily so. Charlemagne started attacking the Scandinavians under the Christian banner, and then out of the blue Scandinavians started attacking Christian facilities. Is it unreasonable to discern a link? You mess with someone, force them to convert at the point of a sword, sometimes they fight back.