Sorry but you are doing something wrong. I bottle conditioned many batched of cider before I started kegging, and I still naturally carb in the keg and it doesn't have any strange taste either.
Bottle conditioning makes weird tasting cider anyhow. If you cant force carb it, dont bother.
There are lots of wild yeast and other living things on the apples that can wreck a batch. Before your next batch use some Camden tablets to kill all those little nasties a day or so before pitching your yeast. Its a tablet per gallon. This makes your odds much better for a good outcome. I personally wouldn't try the spontaneously fermenting method with more than a Gallon or 2 per batch, that way if it goes bad you lose one or 2 instead of it all.for you cider experts we make our own cider on the farm got a small hand press and a small orchard 20 or so trees. So ive made 100s gal of wine and they always turn out great. I've left cider in my fridge for 3-4 months and made some awsome hard cider so... I thought id try a couple bigger batches. did one in a 6gal glass carbouy and one in a half keg (smaller barrel). the one in the cabouy i put brown sugar and yeast in the barrel i left plane. They both taste awfull like vinegar? can any one tell me were i went wrong or what i nead to try next year?