It's sort of like gimbal lock: when a particular social movement becomes so pervasive and powerful the mechanisms of government become dangerously aligned and you lose the ability of the political system to perform an important function, namely defend itself and minorities from the tyranny of the majority. Remember, part of the design of separation of powers is that institutions must retain the ability and motivation to compete for power. If the motivation is gone the ability becomes at risk--either by atrophy or by one institution making power grabs that go unchallenged by others institutions or by the people. Discord is, to an extent, a feature, not a bug.

Hugo Chavez was a nationalist that was popularly and fairly elected multiple times until, eventually, the elections weren't actually fair anymore and the institutions of government were irreparably corrupt.

Nationalist movements aren't the only the way this can naturally occur, but it's perhaps the most common in the past two centuries.