P.P.W9's and the goal of more HP

Cleaned up the heads and made an interesting discovery. What is causing 5 of the intake seats to erode? Is it water ingress and cavitation like seen on diesel cylinder sleeves. If it is where is the water coming from? Porous casting? Or is it the condensation and the hygroscopic nature of the alcohol pulling moisture out of the atmosphere? Or do I have it all wrong. J.Rob