As even your Wikipedia article mentions, even though the east of Ukraine hasn't always been in the same page politically, this separatist movement is fairly new and probably fueled by Russia since at least the 2010s and exploded after the Euromaidan protests .
You can see it by checking the Ukrainian Referendum of Independence of 1991.
Only 12-13% in those oblasts voted no, compared to the whopping 42% from Crimea, which is understandable because it was originally Russian territory ceded to the Ukranian SSR by Nikita Khrushchev as a gesture of goodwill (not that it justifies the 2014 annexation it suffered by Russia).
Even though the vote was about leaving the USSR, we can't separate the Russian question completely from it, and it was an important issue during the referendum.