Check the wall behind your design wall, maybe? Could you possibly have a leak of some kind trapping moisture back there? It probably is humidity in the air, but the fact that the pins in the package didn't show any rust makes you wonder a bit.

I use the same kind of design wall, and I use pins -- cats, airflow from window or fan, lots of tiny seams-- all contribute to occasional block fall.

I've had a problem with my pins in my design wall also. It's make like yours....with the insulation board/flannel covered. I don't notice rust but they do blunt after a while. I don't like that & wasn't sure if they were just bad pins or the board that was causing it. Now that I read your replies to your thread I see that I'm not alone.

My wall is also from insulation board and flannel and I don't have a problem with pins rusting so far, and here in Alabama high humidity is the norm.