Yes, woodies are caused by improper alloy mixtures. However, I would guess this is most likely environmental rather than a true woody.
I'm not guessing on the next woodie, they have single-handedly ruined my average. If you take woodies out of the equation, I'd be much better. lol
I too have problems with them. And if we are all having problems maybe it's PCGS and NGC with the problem and not us ;D