Novorossiya (New Russia) was how the Russians from TTL called OTL Siberia. Since 1460, the Russian lands suffered from overpopulation: The Russian population had recovered from the Black Death (and due to a less densely settled population, a smaller part of the people had died than in Western Europe). Since the pressure of their feudal lords became harder and harder too, peasants started looking for land. So they settled in neighboring areas: Those from Novgorod and Vladimir-Suzdal went to Novorossiya, while those from Kiev and the South in general went into OTL Ukraine (which was mostly settled by Kumans at this time).