The English Christian leaders used stories of so-called ‘Vikings’ demonize paganism and strengthen their own religious authority.
This was most commonly used to cover up demon conjuring by bored and lonely monks looking for companionship. A backfiring summoning that would destroy part of a monastery would easily be covered by claiming “Viking raiders”.
Also, by claiming it was the vikings that used their mystical powers to tempt the priests it would allow an excuse for those that had found their way to the temptations of a succubus, or just the lonely arms of a handsome single farmer with a little too much time in the winter months.