(HT: AK). It's always tempting to speculate and mind-read. Rutland's guesses are as good as yours or mine!
This explanation for Putin's behavior seems to me the most likely:
Grand strategy aside, maybe one can find a more mundane explanation for Russian behavior. As things were falling apart in Kyiv, Putin had to be shown to be doing something – anything – even if it did not make much sense from the point of view of Russia’s national interests. The military and security services had some contingency plans in their office drawers – to secure the Crimean peninsula, and to trigger an ersatz nationalist uprising in the Donbas.
By Peter Rutland
March 5, 2014 | Transitions Online
UPDATE (25.03.2014): Here are more clues to Putin's thought process from the Guardian, with speculation from a few Kremlin insiders: "Ukraine and Crimea: what is Putin thinking?"