i disagree with this. ukraine has been part of russian imperial rule for a very long time, both in the time of the tsar and in the soviet era. lots of russians view ukraine (and belarus) as rightful russian clay, and have a strong desire to restore russian rule over the country. it seems they were mostly content with having ukraine as a nominally independent country as long as its ruler is strongly pro russia, which was the case with yanukovich, but then euromaidan happened and replaced the pro russian government with a bunch of anti russians who are not even trying to hide their desires to align the country with EU and NATO. this is a big slap in the face for those who want ukraine in the russian sphere of influence, one they are not just going to accept.
call it irredentism, imperialism, revanchism, whatever, but it is very real. many in russia view the idea of losing ukraine to the west like americans would view losing alaska to china, hawaii to japan or texas to mexico.