There are quite a few fields where this happens, for a variety of reasons.
Fields where non-management employees are hourly/nonexempt, and supervisors are exempt, very often see the supervisors taking less per hour than the non-management employees - and sometimes less, period, if overtime is common for non-exempt employees. Retail and fast food see this all the time; some unionized jobs where the hourly/nonexempt staff have regular overtime will also see this. Why do people become supervisors in these fields? Of course, it's the hope to become higher level supervisors or managers for some; for others it's the stable hours and wages where nonexempt staff might see part-time status; for others it's the kind of work - a preference for supervisory tasks to manual labor, for example.
Fields where the non-leadership staff are highly skilled and hard to find, and supervisors do not need the equivalent level of skill. I'd even say this translates some to developers, where a highly skilled developer almost always makes more than the project managers theoretically managing their day to day work (but not necessarily the actual "direct report", depending on the structure).
There are probably others; it really boils down to supply and demand, and how much responsibility the supervisor has. If the supervisor is just a line worker with a key, they probably make a bit more; they wouldn't ordinarily qualify for unpaid overtime in the US (I didn't, in retail, for example), and so they're compensated for that responsibility. But if the supervisor really has less useful skills than the line worker, or if they can be defined as exempt, it's just a matter of whether they can convince enough people to take the job at less effective pay, either with the carrot of more actual pay, of later gaining additional earnign potential, or the stick of less consistent earning potential.