Hi there it seems to me a moisture problem, and no airflow being locked in a zip lock bag.
Secondly having them loose where they can contact each other positive to negative is also not a good way of storage.
If you want them in a zip lock bag put a small moisture absorbent sachet in the zip lock bag as well.
I store batteries for years and if you do it right there is no problems.
My two cents worth.
Leon
