Kids will be kids
On a more serious note looks like it could be a case of Polymer Degradation. To quote something I've just read
"
The degradation of plastics can be said to begin as soon as the polymer is synthesised, and is increased by residual stresses left by moulding processes. This can be followed by exposure to light (especially UV), humidity, oxygen, heat, bacteria and stress. Plastics can also be contaminated by other materials, including other plastics."
Maybe this card has been stored face down (with the figure resting on the bubble) for a long time and over the years the 2 different plastics have reacted in a negative way.