Actually I bet they're playing the new expansion.
All those new trade mechanics I bet there is a war declaration to force ship basing rights / trade rights. Russia was worried that Ukranian peasants were going to change the government type and align them more with western governments, and then cancel ship basing, so they broke stability in order to enforce ship basing rights or something. They're just occupying while they consider the AE from just taking the provinces for themselves.
All those new trade mechanics I bet there is a war declaration to force ship basing rights / trade rights. Russia was worried that Ukranian peasants were going to change the government type and align them more with western governments, and then cancel ship basing, so they broke stability in order to enforce ship basing rights or something. They're just occupying while they consider the AE from just taking the provinces for themselves.