This is how Russia received its name. It was a Scandinavian import like ours is an Italian import. The Rusi Rurik and Amerigo Vespucci have that much in common. Russia should really be called Eastern Slavonia and America should be called Indiana. But, unfortunately, history and those who make it are not always particularly conspicuous for scientific logic.
Nobody knows when or how the slaves separated themselves into West, East, and South Slavs.
And certainly no one knows why this division occurred. In the fifth and sixth centuries we read in contemporary accounts about Slavs existing and in the sixth we have evidence that they lived between the Carpathian mountains and the Dnieper river. They settled mostly along river basins. They were held together by a vague kind of religious mysticism which gave them tribal consciousness. According to the monks who wrote these chronicles the event took place in the year 862.