You have to remember that víkingar weren't a homogeneous group, and that Britain was never invaded by víkingar, but rather by Danish petty-kings. The distinction here is that a víkingr is a pirate, a criminal, and thus operates outside of the bounds of normal society. They would raid Scandinavian villages as readily as foreign ones, and would do so because there was no real sense of national identity and solidarity to prevent them from doing so.

My go-to analogy when people ask about víkingar raiding Scandinavia is this: The Mafia and N'Dragheta are Italian criminal organizations, but still operate in Italy and target Italians. The ethnicity of their victims doesn't matter to them, only the potential for profit and increased social and cultural capital within their organization.