Unfortunately Matt, it sounds like the plastic is breaking after all these years and it's just starting to crack and crumble
I bought a NOS board of a mate of mine, still in the original wrapper, and left it on the wardrobe for a very long time. When I came across it I thought, oh, and grabbed it off. As I lifted it up, the thing cracked and crumbled before my eyes
I couldn't believe it
As it was now fooked, I tried it in my hands and most of the board just crumbled so I just put it down to the plastic has given up the ghost
Hope it's not the case with yours but sure looks that way