1) Viktor Yanukovych attempted to bring Ukraine under Russian influence. The people in Western Ukraine naturally revolted, as they want to be part of the Western alliance.
2) Crimea has a majority Russian population. They support Russia, and welcomed the Russian invasion.
3) This is a situation where you had 2 different ethnicities living under one roof, which was an unstable situation that could not last.
4) Viktor Yanukovych and his henchmen were Oligarchs who looted the treasury and turned it into their private piggy bank. The new leadership in Ukraine consists of Oligarchs who are looting the treasury, and turning it into their private piggy banks. Some things just don't change, despite change.
How the hell can people claim that the CIA was behind this? There is absolutely no evidence to support that. The facts point to the obvious reason. Western Ukranians, and Crimean Russians could never stand each other. Something had to give.