It's a good question. The very concept of social contract theory doesn't really jibe with a purely voluntarist ethic. If somehow, every single member of a polity wants to secede, it's hard to find anything wrong with that; while if 50%+1 want to, that seems unjust to the remaining ~49%.
But suffice to say, it's an open question what threshold of popular support justifies something on the order of California seceding the union. Voluntary association (and disassociation) is merely an ideal to strive for, and we will always fall short of that ideal in the real world.
There are historical instances of peaceful, voluntary disassociation of democratic states; the "Velvet Divorce" between the Slovakia and the Czech Republic comes to mind: https://en.wikipedia.org/wiki/Dissolution_of_Czechoslovakia
But suffice to say, it's an open question what threshold of popular support justifies something on the order of California seceding the union. Voluntary association (and disassociation) is merely an ideal to strive for, and we will always fall short of that ideal in the real world.