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?
(An exception was the 2006 referendum in Montenegro that gave it independence from Serbia, which passed with only 55.5% of the vote. This seems to reflect the general principle that Everybody Hates Serbia -- if you don't believe me, just ask a Serb. Of course, the events subsequent to June 28, 1914 did generate a little ill will toward Serbia.)
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: