That looks like severe dehydration.

An opened (as in the inner seal was broken but the lid was on) jar of coffee in a sun-facing cupboard for a few months here in England was enough to get some instant coffee I had into a much darker, browner state. Threw it out. A completely sealed jar of the same still looks OK through the glass over a decade later. I should probably throw that old jar out too, but I digress.

The dehydration you have there looks like some carbonisation has taken place. That only really happens with high temperatures and/or having a desiccant / actual dehydrator nearby, but I wouldn’t be surprised if (natural) chemicals in the coffee have reacted with each other and the air over time to generate acids and the like. This in turn will have allowed water to be more easily broken out of otherwise non-volatile compounds (things like starches and sugars) and carried off with the air.

i.e. it’s effectively burnt.