yup, metal cans
Then that was likely it.
Detinning.
Especially if the coating was compromised in some way, just a tiny scratch, or imperfection from the coating process.
When I do home canning, much as it frustrates me to waste canning jar lids, I am anal about inspecting them for scratches.
If there are ANY at all, I mark them with an X in sharpie and they go in a separate container for use only in the freezer, like for jars that are storing nuts, beans, rice, or spices. I freeze them all and they keep about forever. They have a long shelf life to begin with, and when frozen, keep even better.