Occam's Razor tells us that the Russians acted to 1) stop the constant bombing and cultural suppression of the Russian speaking people in the east and south of Ukraine that had been ongoing since the US backed coup in 2014 and 2) to finally move against the build up of forces and weapons that the US had been pouring into Ukraine since the coup to trigger a war on Russia's border that they hoped would lead to the destruction and dismemberment of the Russian state itself (the openly stated goals of the US).

(In parenthesis, I will also note the very silly but oft-repeated mistake of conflating Putin with Russia in the comment above. Russia does not act on the whims or personal ambitions of Putin; indeed, Putin stays in power largely by responding to the needs and desires of the Russian people, and the pressure inside Russia for him to act against the existential threat building on the Russian/Ukrainian border has been very strong and he has faced criticism for not acting earlier.)