... following promotion a person is likely to regress to their baseline competence, losing that extra something that prompted their rise. That baseline might be above or below the degree of competence demanded in the new, high-level job. If in a particular workplace the staff who are promoted consistently fall short in this respect, promotion can become the dominant force driving pervasive ineptitude [...]
More in this New Scientist article.
Hat tip: Teppo Felin at OrgTheory.