The Crimea, presently, is threatening to secede from Kiev. That this appears to largely be the work of Russian special forces is a moot point.

Russia is interpreting its obligation to protecting Ukraine's territorial integrity as compelling it to prevent the Crimea from seceding. In its eyes, the regime in Kiev is illegal and antagonising ethnic Russians in the Crimea. The simplest solution, to Moscow, is fighting the illegal" regime.

This follows a similar script to Georgia - provoking the new regime into aggression and then intervening to keep the peace (with such an intervention not necessarily restricted to the initial separatist areas, e.g. South Ossetia or Crimea). In Russia's defence, it is a strategy the United States has used to great success as well.