Outcasts or whatever are painted bretons camping outside Marcart. Care to elaborate about 'backstory making perfect sense'? They are not and cannot belong to Skyrim, and their concept is unfeasible.
The Forsworn or are the original inhabitants of the area, or at least were there before the Nords. They have a different culture and religious believe then Nords, and are actually closer to Bretons in term of race (though their culture is completely different).
It makes sense because that's simply the history of the area, and because it's fun to see. They fight for their independence the same way the Stormcloacks do, but of course Ulfric and the Empire heavily disagree with the forsworn. The reason why they are so active now is because some 20 years ago or so they had a good chance and took over Markath. They lost it again later, but it has revitalized their campaign so to speak.