coping channel rotting away

I have a 4 year old IG/V 37,000 gal. pool. I just noticed that a 4 foot piece of coping on my pool is beginning to rot away right where the liner hooks into it. The weird thing is that this is the only piece of coping in the entire pool that this is happing on. Any idea what would cause this, and would this be a warranty item?

Thanks.

Any chance the pool deck in that one area slopes the wrong way, i.e. towards the pool? Or a "birdbath"/ low point in the deck?

Nope. The coping is made out of aluminum, and the channel in the coping that holds the liner in is just disintegrating away. The deck is level all the way around, and the water level doesn't even come close to reaching this. I would think it must be some form of chemical reaction that is eating the aluminum away, but you would think that if that was the case, that this would be happening to all of the coping and not just this one section. One other note: The pool does have an auto cover, so it is covered most of the time.