We once had a rapid loss of pressuer (well, not explosive, but above 2000fpm in the cabin and not controllable) during cruise in a 737-3 without any apparent structural problem or pack problem. An emergency descent was done according to standard boeing non-normal procedures and the flight completed without further problems.
In the post event check two failed pressure seal clamps on the apu duct (one at the aft pressure bulkhead, the other in the wheel well) were found which were not covered by the boeing mainentenance planning manual. The one at the bulkhead was out of position and the one in the wheel well was broken. In a fleet inspection on those pressure seal clamps nearly all of them were found to be installed incorrectly right from the production (back then the fleet average age was around 2 years and no D checks done yet).