Telstar - most of the discussion above relates to CPCS (as fitted to the 727-200/200/400/500) You, in a NG have DCPCS, and I believe otherwise the same system as a -400.
Cause is: A rate of descent such that the cabin cannot reach landing altitude by the time the aircraft does. It is a simple equation involving cabin altitude/normal programmed cabin descent rate and time from cruise to landing. If time to landing is LESS than required by the cabin, you 'catch' it, resulting in the neg relief valve opening and the cabin descending at aircraft rate which can be unpleasant for all. EG Prog rate 500fpm. Cabin 'cruise relay' trips at 1500' below cruise alt. Cabin starts at 7500ft, but descent takes 15 minutes = you catch it! I guess your descents are not fast enough if you have not seen it!
With DCPCS the only answer is to increase the cabin diff (=reduce cabin alt) before descent, which is not a good idea.
AFAIK this can only be done by decreasing the 'Flt Alt' setting which then schedules a lower cabin altitude (let's leave 'Manual' out of this
).
Best solutionsi are:-
Avoid high rate descents
If unavoidable, reduce R of D when you see the cabin altitude at half your actual altitude (assuming progranmmed 500fpm cabin rate.)