Perhaps the Russians know the Chinese are about to attack Taiwan and are creating a diversion and a two front situation. Iran to jump in if the US appears to be losing.
Tom Rogan: Russia is moving thousands of military personnel and a great deal of equipment toward the southeastern Ukrainian border area held by Russian-allied rebels. The deployments are likely to portend a new offensive by those rebels
We should also note that Russian aviation forces have also been reinforced in Crimea. This means that the Ukrainian government-held cities reaching along the Sea of Azov, from Henichesk to Mariupol, are now heavily vulnerable to encirclement and rapid seizure. Such a seizure would give Russia dominion over the Sea of Azov and contiguous sea control right up to the Kerch Strait. It would also provide a Ukrainian land bridge from Crimea to the Russian border, without reliance on the Kerch bridge.
https://www.washingtonexaminer.com/opinion/decoding-russias-military-buildup-near-ukraine