This is a bit of a puzzle.  Two things are necessary for Byne's to occur
- moisture and a source of acid vapors.  Cheap paper, like that used in
making cardboard boxes, can release acid vapors.  However, plastic bags
(assuming they are well sealed of course) should provide a barrier to
both water vapor and acid vapor. Again, IF the bags were well sealed, it
would seem that whatever caused the damage must have been sealed inside
the bags along with the shells.  Here is one theory.  If the shells
contained some water (recently cleaned, or recently collected?) and also
some organic matter (bits of snail tissue), the acids of decomposition
released by the decomposing tissue could serve as a source of acid
vapors, while even a couple of drops of water, permanently sealed inside
a plastic bag, could provide ample moisture for Byne's to occur.  Of
course, if the plastic bags were not well sealed, then the cardboard
becomes the primary suspect, and the box must have been stored in too
humid an area.

Paul M.