Probably Putin became very worried when President Yanukovych was ousted (anti EU/pro Russia) because he's afraid the Ukraine will move further towards joining the EU and if that happens he'll never be able to claim back Crimea and he'll have NATO on the border as well. It's a now or never situation for him I think. I imagine It's all propaganda that there are Ukrainian ultra-nationalists who want to suppress the Russians living in Crimea which is his excuse for sending in the military. 

I do feel very sorry for the Ukrainians (and Russians) who just want to get on with their lives and make it a better one for themselves.