Some element in the glaze must have been unstable.
Raku firing can create the unstable glaze effects, which are due to the glaze color forming in an oxygen-deprived environment. Over time, the components of the glaze that were created from the lack of oxygen begin to pull oxygen out of the air, and change their color. So those beautiful coppery, irridescent "oil slick" colors on some raku ware turn to rather plain colors. There's one on my refrigerator right now.
It was glorious, two years ago.
Anyway, your pot isn't raku, but the example shows that color within a glaze CAN change due to outside factors. The acidity of the mulch probably had a lot to do with it. It would be interesting to find out what happens if it is re-fired to its original temperature.
Joanie