One can easily become an accidental manager, no matter what field you work in (and libraries are no exception). You just do a good job, and one day your boss (or supervisor) says, “you’re in charge today.” Congratulations! You’ve just become a manager. (Good luck.)
Just as easily as you can become an accidental manager, a boss (or supervisor) can create an accidental manager. It just depends which side of the table you’re on. (See above.)
Don’t argue with me. Yes, in my example, the person may have “become” the manager for a day. But almost just as easily, managers can be created more permanently. It is often done with the best of intentions, but no one can really just become a manager intuitively.
But you can do better. If you “become” an accidental manager, tell your boss you need training and clear directions first. If you “create” an accidental manager. . . well, no, I take that back, don’t create one! Groom your employees to become managers first!