The boos started for Russia because of their invasion and takeover of Crimea in 2014, soon to be followed by their incursions into Eastern Ukraine, and their attempt to push westward.
People seem to forget that Putin's war in Ukraine didn't start 2½ years ago. This war is now over 10 years old, with Ukraine repelling every effort by Russia to encroach deeper within. The straw that broke the camel's back was when he used Crimea and the Black Sea to take the southern part of Ukraine, and rolled tanks into Southern Belarus (apparently with Lukashenko's approval, as if he really had any choice) to take Northern Ukraine.
The EBU sat on their hands as the tanks rolled toward Kyiv; until at least 9 countries (including Iceland, Ireland, the Netherlands and the rest of Scandinavia) gave the EBU an ultimatum: Either Russia goes, or we go.
The EBU had no choice, as surely more countries would have withdrawn from the 2022 contest, possibly forcing its cancellation. Russia was kicked out. They soon withdrew from the EBU. By their association with Putin's war, Belarus has not been invited back.