Every normal man must be tempted at times to spit on his hands, hoist the black flag, and begin to slit throats. It is inaccurate to say that I hate everything. I am strongly in favor of common sense, common honesty, and common decency. This makes me forever ineligible for public office. H.L Mencken
This issue was forced by the US. Likely it was the result of being frustrated after being outmaneuvered by Putin when he offered Yanukovych a better deal that was accepted. So the US then calculated that it would pressure Yanukovych through protests, threat of financial sanctions, and by applying pressure to Akhmetov. The opportunity presented itself to remove him and the US took that opportunity knowing full well that it would be of grave concern to Putin to have been cut out by force in this way. Now this is where the BIG mistake was made. What they likely calculated was that they could do this as long as they offered Putin some sort of deal that would allow Russia to have some influence in the government they were planning to form. Over and above that, Putin was likely offered some assurances with regards to Sevastopol and possibly some no NATO guarantees. They thought this would be enough to placate him. And that was the big mistake, because, as Putin indicated in his speech, they have lied and engaged in backbiting on the Russians in the past. So there was simply not enough trust on Putin's part in the US. And he had very good reason to calculate in that way. Then Putin, being a Russian nationalist, calculated that he was not going to take a risk having Russia suffocated by trusting the US. So he enacted plans, that had likely been formulated long ago to seize Crimea.
That's the most likely explanation, IMHO.
No doubt about it.