From my understanding of my college chemistry courses, BD is chemically self-sustaining in the presence of water, oxygen and chloride ions.
PVC provides the chlorides, the air provides the water and oxygen. Chloride ions and copper make cuprous chloride which reacts with water and oxygen to form cupric chloride / cupric hydroxide (blue-green BD) and hydrochloric acid; the latter reacts with copper to make more cuprous chloride, and so on. The chemistry is such that it would keep going if the BD is internal to the coin and/or if the coin is in an enclosed container, but, as many collectors have experienced, BD, or rather the hydrochloric acid vapour generated by the BD process, can also spread to kick-off the process in nearby coins, and given that the mylar
flips won't be totally sealed, I suspect that's what's happened. I'd consider whether the blue plastic storage boxes may be the original source of the chlorides, perhaps containing some
PVC, enough anyway to keep the process rattling on once started.
I wouldn't necessarily shoot the mylar
flips - perhaps they are a source of chlorides in surface coatings but, if archival, they may be innocent bystanders whose only significant role may be to provide a closed container to enable the reaction to continue. But if it happened to me I would probably change out all the adjacent
flips out given that you don't know what the
flips now contain in terms of BD residue, and I would probably follow appropriate advice on BD cures (a subject I don't wish to comment on,
ref. elsewhere on
Forum). I'd consider finding an alternate inert container than the blue boxes. I'm assuming the blue plastic boxes are slide containers; you may have no idea what they are made of (might be full of
PVC or related compounds). Many collectors/dealers use cardboard containers.
