I was aware that they recommended rotating any water you store for emergencies, though it does seem odd. But I'm especially surprised that purchased/sealed water
would go 'bad'. I just would not think there would be enough 'stuff' in there to feed any microorganisms to a degree that you could see/taste/smell the effects. And while it would not be 'sterilized', it should be very sanitary, and there would not be much in there to kill off in the first place. I wonder what's going on?
If it were just minerals coming out of solution, I would think that would just be some small amount of sediment, and I wouldn't expect it to taste bad.
I know there are bacteria that feed on iron in water, but I don't think they cause taste problems in small quantities (I wouldn't expect bottled water to be high in iron, it really doesn't taste good to most people), and I don't think they are dangerous either.
I've got about about 25 gallons of water stored away in the basement, fortunately it has all been boiled, and contains some alcohol and hop oils and the pH is low enough to keep it shelf stable for a long time. I also am good at rotating and replenishing my stock on a regular (~daily) basis. Tonight, I think I'll have some 'Koelsh water' with dinner.