Looking at the Wikipedia page on "Independence Referendums," I note that successful secession votes tend to be extremely lopsided. For example, Norway's secession from Sweden in 1905 received 368,208 votes For versus 184 votes Against independence.
I'm not sure why this pattern is so common: Cheating? Bandwagoning? Boycotting by sure to lose sides? Fear of reprisals by winners? They don't hold secession votes unless they are likely to win big?
Here are the percentage Yes votes for secession in the Ukraine on December 1, 1991. You can see that Crimea (54% yes) and Sevastopol (57% yes) are definite outliers, while no other oblasts were under 83% yes. So, Crimea seems like a special case: