The Norse in Scandinavia were sick and tired of the shortage of farming land and a population boom.
So, Norse traders traded with the Anglo-Teutons in Britain and realized the power of the Teutons.
And they also traded with the Slavs in Russia and saw how they were not so strong.
Then it happened. The Norse had raided Novgorod. For years the Norse had raided Slavic settlements in Russia.
Then the Norse became more ambitious. They invaded Novgorod and captured it. Many Russians were enslaved and the Norse burned down Novgorod and Norse warriors brought their families. Over time Novgorod became a Norse city and the land around it became Norse farms and villages home to Norse clans who used to rival one another and then became friends in Russia. Only Novgorod and the land around it was Norse. The rest of Russia was Slavic.