well vikings (vikings=Nords) had three ways of burial traditions
Vikings are one of the inspirations for Nords, but they are not equivalent.
According to Bloodmoon, Nords need to be buried in Skyrim. Those warriors with tombs on Solstheim are those whose bodies could not be returned. Since this was back in the early 1st era, I'd say the barrow burials are a pan-Nordic tradition. I don't know whether they survived several thousand years of cultural revolution.
Bloodmoon also mentions ship burials and "burial grounds."
The Skaal consider the Stalhrim to be holy. During the great war with the Dark Elves, many heroes fell in battle. Some could not be returned to Skyrim, and were buried here. Great magicks were worked on their tombs to protect their belongings from grave robbers, and their corpses from worse things. Energy was drawn from the land itself, and our heroes were encased in tombs of ice. That ice is Stalhrim.