Immigration to Scandinavia was exceptional during the Viking period

> Gene flow from the British-Irish Isles during this period seems to have had a lasting impact on the gene pool in most parts of Scandinavia. This is perhaps not surprising, given the extent of Norse activities in the British-Irish Isles, starting in the 8th century with recurrent raids and culminating in the 11th century North Sea Empire, the personal union that united the kingdoms of Denmark, Norway, and England. The circumstances and fate of people of British-Irish ancestry who arrived in Scandinavia at this time are likely to have been variable, ranging from the forced migration of slaves to the voluntary immigration of more high-ranking individuals such as Christian missionaries and monks.

Also perhaps relevant:

> the results shown in Figure 5 provide at least tentative evidence that gene flow into Scandinavia of eastern Baltic ancestry and, to a lesser extent, also British-Irish ancestry was female biased.

This might be overseas Scandinavian males coming home with new wives. Or sex slaves/concubines. Or something else. Ask a historian.