And what was causing the fermentation? Microbes of unknown type.

That can might not have been sterilized properly when it was packed, or it might have developed a tiny leak and become contaminated. There is no guarantee that the microbes that caused the explosion are the same harmless microbes that turn cabbage into sauerkraut.

Never mess with a can that is swollen or under pressure.
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I was in a store one day and overhead a man complaining to the manager about a can that was dented. The man said, "I've worked in the food industry. This dent can cause the tiny leak where the side joins the lid, and germs can get in there and spoil the food." Since then, I've refused to buy a can that's dented.