I work at a company where employees get to choose three-weeks sabbatical (or paid equivalent) after reaching 7 years of tenure. Of those I know who took the sabbatical, a good proportion of maybe 20% to 30% come back off their break and promptly hand in their resignation.

Talking to my former manager who did this, he said that he never realized just how burnt out he was until he had some time on his hands at home. I think some folks just exist in a fog of burn-out and don't really notice it until they get a chance to stand back and evaluate their situation.