As a consumer, I sometimes notice an odour to chicken a couple of days before the BBD which screams coliform contamination to me. Have you checked the evisceration process? I'm not familiar with meat processing but I would guess if there is a significant cross contamination it could lead to early spoilage? I've no idea if the carcasses are cleaned or processed in any way post evisceration or whether you rely on good technique?
Also I would, if you don't do it already, keep some packs of each production run in a refrigeration unit which mimics consumer fridges (ie 8 degrees is on a good day!) Then check the packs yourself.
Are you gas flushing? If so, you'd be surprised how many microleakers you can have in seamingly intact packages. You can get fairly good leak checking equipment which detects CO2 to check for these and some are now available in line, although they're not always fast enough (yet) for modern processing.
It is a possibility, especially in this weather that it could be down to poor storage at stores and with consumers but unfortunately that's not an excuse which washes with the retailers and also it might just be highlighting a contamination / leak issue which is being hidden at other times of the year. It might be worth looking at the data and looking for seasonal spikes anyway just to give yourself some breathing space but I wouldn't stop even if the data strongly suggests a summer effect.