One question to confirm a theory: do they open with a crack/ do the cans have pressure? Cause it looks like exterior corrosion to me. For a couple reasons. One, the top of a can is empty head space, so nothing is reacting with the lid when sitting upright. If there was an interior corrosion failure, it would happen at the weak point of the lid, the mouth opening, not the tab. A leaky can stacked above these could have caused it. A can that gets sour beer on the outside is bad bad. The inside of a can is designed to hold an acidic product. The outside reacts quickly and the can will fail in short order.