I know somebody – a tenured professor – who routinely teaches 5 and once even 6 courses instead of 3. For no extra pay. The reason why she does this is that every semester she’s told “If you don’t do it, there’s nobody else to cover these courses. The majors won’t be able to graduate and the program will die.”
This is not an isolated case. Many tenured academics teach above their contractual load at no extra pay because they are told that otherwise their program will be closed down and they will be fired.
Literally, the only thing that can prevent this is unionization of tenure-track and tenured professors. An administrator will not make these threats in front of a union rep who, of course, will make sure the conversation is recorded.