Many factors can affect film stability. The most common problems arise from an interaction between the chemical structure of the original film base, the way the film was processed, and the storage conditions (temperature, humidity, housing materials, and degree of air exchange between the film and its housing and larger atmosphere). The negs in the different formats probably differed in some of those variables. Some of the early safety films are prone to a decay condition called vinegar syndrome, and they do smell like acetic acid when the decay is advanced. If the 5X4s have this problem, then it is very important to segregate them from the other stable negatives (like not in the same cabinet), keep them in housings where they can "breathe", and copy them as soon as possible.