Because Russia is failing apart. People living in post Soviet sphere see how other countries are developing. Ukraine had higher GDP per capita than Poland - now it's 4x lower. It's true for every single place from Kaliningrad, Caucasus to Vladivostok. They are worse off they their neighbours for purely political reasons.

Only thing that holds everything together is the authoritarian regime with strong military and natural resources. The problem is the world will become less dependant on fossil fuels and mining in Siberia is more expensive than pretty much everywhere. Russia won't be able to pay for the army.

This is an existential treat to the Russian state, hence the war is very likely.