Could it be an adhered veneer installed in compliance with a previous
edition of what is now 1997 UBC Section 1405.3?
>From your description, I'm visualizing that the slate is applied with
bedding planes vertical, and the veneer is separating due to delaminations
along bedding planes within the veneer-stone units, while the adhered layer
remains attached. Is it something like that?
If moisture can get to the edges of vertical bedding planes, it may be
penetrating into the stones along bedding planes and damaging the binder
that holds the laminations together, leading to defoliation of the stones.
I'm not sure that I've got the picture right -- I'll wait for some feed-back