Scale for one thing. Two, out of the blue military exercises. As the Ukrainians military forces get better equipped, the bloodier it will be to stop them from pressing into Dontesk eventually, and impossible for the Russians to deny involvement. If Putin wants a cooperative buffer state he’s on a timer anyway. So, go in now while it’s easier, or wait two years and suddenly have to deal with a Ukrainian military capable of overwhelming the forces on the border the Russians will send in to try and defend Dontesk. It’s do it now, or lose and never get a better chance since you couldn’t do it earlier.