Peter Zeihan had an interesting take on this, that the Russians moved on Ukraine because their demographic decline means that if they don't invade now, they will forever lose the ability to wage war and hence defend their homeland. The Soviet-era population is rapidly aging out of military service age, and the post-Soviet generation is tiny, and so they will not be able to field a meaningful army if they wait. Basically Ukraine is a last-ditch attempt to save the Russian state.
https://www.youtube.com/watch?v=5-r7z0lM5_k&t=42s
Interestingly, he predicted the Ukraine war 5 years before it happened:
https://www.youtube.com/watch?v=rkuhWA9GdCo