Indeed, the non-aviation community does not understand the need for mayday in this context.
The unauthorized departure from assigned altitude is in fact a mayday, even if the aircraft is not spiralling into the sea. The mayday is issued because the immediate threat to life is any other aircraft already in the FL descended to, in the area, and the people on board the transmitting aircraft.
Completely normal, and completely logical.
No need for alarm here, folks...