Suddenly, the new supervisor is now charged with a whole new range of responsibilities, many of which have little to do with technical expertise.
Managers often deal with great deal of paperwork and people.
Although paperwork is usually the most tedious, it's often the most predictable. People aren't predictable. They have moods, illnesses, career expectations, crises in their family lives, etc. The supervisor's technical expertise is often useless when it comes to supervising people. The supervisor is responsible for signing time cards, authorizing overtime, granting compensation time, dealing with performance problems, developing job descriptions, following hiring procedures, dealing with grievances, conforming to a complicated pay system, and the list goes on.