Cabinet Card Foxing?

My N173 Ward has some foxing and I have seen other cabinet cards with this condition. I was wondering if Foxing is caused by a bad chemical reaction in the photographic process or something that happens later due to outside influences.

The foxing happens over the years and not during the initial photographic process. I think it's pretty typical wear & tear that you tend to see on photographic cards from that era. Possibly due to exposure to the elements, or from rubbing against something that was stored on top of the card. Your N173 Ward has a great image nonetheless, Jerry!