excess hcl is the obvious answer, and easy to remove...however, swim once dreamt of some pristine gear that was absolutely sparkling, and put it into a tiny zip-lok baggie, and it started to get yellow over the course of a few days.
after about a week, it was extremely yellow!
didn't effect the quality at all, yet swim remains baffled as to what had occurred, and never saw a repeat performance.
had to assume it was something in the plastic bag that caused it.