Not an unusual problem on the NG. I would bet money that they were flying at the top end of the altitude capability of the aircraft i.e. above FL 370 ish, where the higher cruise speed of the NG i.e. .78 or more, brings you much closer to the MMO of the aircraft. If correct programming of the descent is not carried out, such as descent forecast winds etc, then as the aircraft retards and starts the VNAV Path descent, and goes high due to a strong tailwind, the aircraft will attempt to follow the path and pitch down to the MMO before Overspeed Disconnect will show.
The MMO protection to reduce thrust will not work because the thrust levers are already at idle with retard/arm showing on the FMAs.
With the first UK operator of the aircraft, we artificially put in a descent speed of .78/280kts to cater for just this situation, which caught us out as well. With a lower cost index that my current company are using for max efficiency, and due to higher fuel prices, the descent speed is much lower, but the Mach is still .78, which is still on the descent page.
I teach crews to be aware of this situation, especially with a strong tailwind component, even if the descent has been fully programmed, and be ready to go LVL CHG with speedbrake hand at the ready. Another way would be to programme for a VNAV SPD descent and keep an eye on the descent profile, adjusting as necessary till below about FL340 or so when the gap between descent speed and MMO should be sufficient to allow for a Path descent.
Hope this helps