The reason is it so easy to exceed VMO when descending at MMO is the aircraft is accelerating more quickly than does at normal descent Mach. In order to maintain MMO, the pitch must be constantly decreased. If you wait until you reach VMO to transition to constant indicated airspeed, it is impossible not to overshoot. You need to lead with an increase in pitch about 10 knots prior to reaching VMO (probably varies by aircraft; this is my experience) . Most pilots will set off the overspeed clacker when flying emergency descents at MMO/VMO.