Correction to my earlier post — Simferopol is the capital of Crimea, not Sevastopol.
Regarding choice, in the early ‘90s, they did choose to become part of Ukraine. They could have easily joined Russia, or gone independent, which they almost did. And a popular vote is/was scheduled for May.
The request for assistance came from the newly-minted Prime Minister, only days after his (probably illegal) appointment.
I’m not saying that Russia has no national interest in Ukraine or Crimea. Obviously, it does. But they had no justification for their recent actions.