I’m not sure why you wouldn’t expect democracy to destabilize non-democratic nation states. You’re moving from an authoritarian system that can squash legitimate grievances and desires (e.g. Yugoslavia) that would otherwise need to be addressed under a democracy.

If you look at history, a jump from authoritarian systems to stable democratic systems seems to happen in either countries had some quasi-democratic system prior (e.g. Poland) or a strong national identity to unite the population (e.g. Japan).

Most other democracies transitioned through decades of instability and non-democratic government when given independence (e.g. South Korea, Taiwan).