When Soviet Union collapsed, it was precipitously broken up along administrative or historical internal borders, without giving much thought to the ethnic compositions of the countries that were formed. Russians who found themselves cut off from Russia went overnight from being the dominant ethnicity to being an ethnical minority. I guess they didn’t like it.
Replace “Soviet Union” with “Austro-Hungarian Empire” and “Russians” with “Germans” and you have a decent description of the ethnic situation before WWII.
Replace “Soviet Union” with “Austro-Hungarian Empire” and “Russians” with “Germans” and you have a decent description of the ethnic situation before WWII.