Maidan put pro-west, anti-Russian fash sympathisers in charge. The population of Crimea, being ethnically and historically Russian, said ‘fuck this’ and left, then joined Russia.
There was various degrees of US support to Maidan and Russian support to Crimean independence.
Geopolitically, the annexation of Crimea was a response to the west’s pulling of Ukraine into its orbit. The Russian preference would have been to keep a Ukrainian govt sympathetic to Russia. But when that proved impossible, taking only the pro-Russian portion of Ukraine was the next best choice.
My understanding is that, historically, Crimea was Russian, and was transferred to Ukraine as a gift during the USSR era.