all I can think of is that some airborne yeast spores may have settled in your beans and started some fermentation---beans have sugar in them and that's all you really need plus a "food" source---throw them out and start over----do the quick cook method--add water until it's at least 1.5 inches over the beans, bring to a boil, turn off, move pot to another burner and let set for 1 hour. Then pour off the water, add fresh water, broth, etc., and then cook as normal.