Perhaps something to do with the speed of sound, measured as an adopted 'standard' at sea level, with temp and pressures of 15 degreesC and 1030mb.
You can fly subsonic at sea level in a WIG, Hovercraft or Kite-Board
However, any rapid acceleration or manoeuvere at about or above 725kts, at low level, or higher altitudes, in these climate conditions, will create a sonic boom.