The "silvering" is the result of compression on the photo surface caused by other photos or things stacked on top of the print. This is one hypothesis I've been told. I'm sorry I can't validate this with any reference, though.
I don't have much faith in that hypothesis. I've seen it happen to a modern RC print which was framed as soon as it was dry, but in a cheap supermarket frame. Over only a few months the dark areas took on a glossy blueish sheen, and within a year the midtones were blotched. That frame was definitely not "archival" - so I kept it hanging on the wall to demonstrate (to customers) the value of proper framing materials!