If you do not have an expensive filter which has sealed edges (here in Germany called "according Käsemann") moisture will creep into the filter and degrade the polarizing layer. But this will usualy happen fromt the edges towards the center.
An completely uneducated guess from me would be, that he thin layers of glass of the filter got under preassure and the polarizing layer between them was destroyed this way.
However you are aware, that with a linear polarizing filter exposure measurement and phase AF will not work properly?
Thus one could say, you now have the unique opportunity to replace an inappropriate piece of gear by the latest, shiny new.