Canned Peppers: Very subtle acidic taste
It sounds like (benign, tasty) lactic acid bacteria fermentation, possibly along with leuconostoc or something. But without the appropriate levels of salt, it's also possible that less friendly microorganisms are also growing.

As a rule, canned food with unexpected microbial activity -- regardless of process -- should always be discarded. Even if it's not botulinum (it's almost certainly not), a jar of peppers isn't worth a week on the toilet.