IRROPS is irregular operations, which can range from weather and air traffic delays/cancellations, flight diversions to a different airport causing lengthy delays or unplanned overnights, crew timing out, all sorts of things that can happen in an imperfect world. In those cases the UM fee more than pays for itself as minors aren't able to rebook themselves or get a hotel room on their own if needed.
SHTF is basically s*** hitting the fan, as in what happens in IRROPS situations