A mob overthrew the Ukraine’s government which meant Crimea was no longer part of an existing national government.
The thug was voted out of office and fled.
You apparently did not pay attention to the build up of the protests. For months people peacefully gathered asking the government (run by a Putin puppet no less) to do what had been promised during an election campaign. Then in an EDICT all assembly was outlawed. Troops and police were ordered to fire on those assembled and many resigned rather than fire on peaceable assembled country men. At that point all hell broke loose.
Read from those in Ukraine and those with family there. Alex Jones and RT are NOT reliable sources
You can read here http://www.worldmomsblog.com/2014/03/10/ukraine-language-culture-reason-war/
Yes, rather an anti-Soviet revolution prevailed in Ukraine. The government in Ukraine is the Supreme Rada, that was not "overturned" and in fact the Rada confirmed the victory of the revolution. This does not cancel the existing constitution, and surely is not a legal ground for any province to secede.
Indeed, the referendum was illegal also per the Crimean constitution::
An act by an authority of the Autonomous Republic of Crimea which is contrary to the Constitution of Ukraine is therefore also contrary to the Constitution of Crimea.
Besides, the referendum in itself was not conducted in accordance with recognized international standards: it took place hastily, under military occupation, was not worded in a neutral way, was not preceded by consultation with all stakeholders.