Extremely straightforward - Sevastopol. Russia's naval base on the black sea. If they lose Crimea to a hostile government in Kiev they lose Sevastopol and their access to the black sea that is hugely strategically important.

On a less strategic level, everyone in Crimea already either were Russian personnel in the Black Sea Fleet or worked for the Black Sea Fleet. Crimea was able to change flags from Ukraine to the RF without any friction because it was defacto already a Russian territory, only aligned with Ukraine on paper.

Russia's formal annexation of Crimea was an inevitable and predictable result of the 2014 coup. Russia simply cannot lose Sevastopol. No nation that wasn't completely subjugated would allow such a critical strategic asset to be stolen without a fight. And of course everyone in a position of power knew this. I won't go so far as to say it was an intentional provocation intended to start a hot war, but I also would not be the least bit surprised if that was a consideration in the coup. And of course all the whining about violations of international law and custom is absurd bad faith nonsense. Everyone in power knows why Russia took control of Crimea, they're just counting on gormless rubes who have no understanding of power to look at the matter in the most superficial and guileless way imaginable, or just huff "Russia bad because asiatic brainpan" as deeply as possible.