Not this shit again. For a board that wants to avoid going the route of /leftypol/ you're well on your way there.
As for the subject at hand: No it was not imperialism of any kind.
See https://leftypedia.org/wiki/Baltic_states* for some details.
To summarize, the governments of the Baltic states of 1939 were extremely nationalistic and pro-fascist, and their government was generally unpopular due to its oppressive behavior (imprisoning jews and socialists) and its ineffectiveness at defending their national borders (ironically). It took Soviet diplomatic authority to halt a Polish invasion that resulted in the taking of Vilnius, and VIlnius was only returned after joining the USSR.
The deposed nationalists of 1939 were given light treatment initially, even as the populace greeted the USSR openly outright voting to join it. This let them become the Forest Brothers of WW2 who were active primarily during 1941 and 1944-45, acting solely against the Red Army, and ignoring the invading Germans in 1941-43. This appropriately received harsh action from the side of Soviet authorities and if you actually read the atrocities the Forest brothers committed (including but not limited to slaughtering entire villages that weren't explicitly against communism) the measures taken were appropriate. The Baltic areas under the USSR were the most well off in the entire Soviet Union and were subject to a lively economy. However in the 90s, as Glasnost of Gorbachv let all the Nazis out of the woodwork to lie on television and the Perestroika's intentional economic failure, the Baltic nationalists took this as an opportunity to muddle the minds of its people with rhtoric of being "independent" of "le russkies". It was all a load of codswallop, but the situation at the time and the disbelief of Soviet news that told of capitalist shittiness led to the modern russophobia, which has become an economic asset for the Baltics today.
TL;DR: Not imperialism, Perestroika and Glasnost let nationalists come back and trick people into revolting over bullshit.
*The wiki needs contributors so feel free to sign up and help write.