Hmmm... just speculating/wondering if the Vikings are a separate theme or if Lego modelled the Rogue Knights after the Vikings.
If Lego is trying to make Morcia as a kind of Gondor, and the Moorlands are a kind Mordor... perhaps these Vikings/Rogue Knights are the "Rohan" of Lego, but with ships.
It makes sense, the Rogue Knights were banished from Morcia, and the only place to live outside of Morcia and the Moorlands would be somewhere really far off that you need ships to reach it. Besides, how much more "rogue" can you get than being a Viking.
But then, I could be wrong.....