Honestly, it was probably just old age! It was coming up on ten-or-something years, and it was fairly cheap to begin with. I suspect it was the pen, but I can't be sure. But I've been very careful with the placement of both the pen and the tablet, keeping them safely tucked away when not in use.

Though I can recommend - if your tablet brand provides it - protective films or plastic screens to put on top of your tablet to keep the micro-textures from breaking down. I've been drawing on a glass-smooth surface for years now.