The problem with trying to pinpoint the cause of PMD and environmental damage is that there are so many, many, many ways it could happen after it left the mint, unless you were there when it happened it's difficult to know for certain. That's why most members just say PMD or Environmental damage and leave it at that.
Oil from our hands, going through the washing machine, spittle, scotch tape, carbon, low ph, high ph, sulfur fumes and moisture, humidity and dampness to name a few.
looks like it sat in the bottom of a moist car cupholder for a few months to me with the different shades of toning, indicates different exposures to liquid at different times but not enough to submerge the whole coin and tone it evenly. Just one of may opinions... lol