Myself and some other managers I know became managers for being competent at our science-based jobs when the company wanted to expand. Our education and career up until this point mostly had not involved learning skills like delegation, teaching, scheduling, and team-budgeting, not to mention the interactive social skills needed to successfully manage individuals.

Some bad managers are just good workers that weren't able to suddenly learn these skills when their employer insisted they manage a team so it could pursue its endless quest for infinite growth by setting up hierarchies of workers. Good managers are either trained in management or extraordinarily talented.