Southern and Eastern Ukraine is very pro Russia and has a disproportionate amount of Russians compared to the northern and western part of the country. How did Ukraine's population become less homogenous?
Crimea has historically belonged to Russia, but in 1954, the Soviet Union, under the leadership of Nikita Khrushchev, transferred Crimea from the RSFSR to the Ukrainian Soviet Socialist Republic.
Because Ukraine was so close politically to Russia at the time, Russia was still able to use it as it pleased. Today about 60% of the Crimea's population remains ethnic Russian.
Part of the reason why there are proportionally more Russians, is because there are fewer Ukrainians than there ought to be.
The Holodomor was a famine caused by Soviet mis-management of food supplies. It was really bad. Like, really bad. Officially, several million ethnic Ukrainians killed. Unofficially, reports of people resorting to cannibalism. Several countries, including the United States, recognize the Holodomor as an act of genocide/ethinic cleansing on the part of the Soviets.
This, combined with the Crimea thing, makes me start thinking that the USSR at that time was going out of its way to undermine the Ukrainian ethnic and national identity. That opinion is on the level of a conspiracy theory, though. Even whether the Holodomor was intentional is technically an open question, and the motivations for causing either of these actions is just guesswork.