http://puu.sh/8V94O.jpg
I made sure there is 5 tiles open space (plus a little) in between the piles and any obstacle.
The piles are in a completely paved, walled off area, it is not possible for any wildlife to get near them.
The pile I just harvested to the left of the one I am standing below gave me 100% well dried boards.
But now this pile is giving 100% moldy boards, at least the first 24 boards harvested were all moldy.
How? I did not come near them during drying, nor did any village member.
EDIT: finished harvesting it. The full pile yielded a stunning total of 3 dry boards. wtf?
EDIT2: According to a village mate, tumbleweeds can jump over makeshift fences, so that could be the culprit. Didn't know that, guess I'll have to start replacing that fence then.