Weird cider carbonation...

This is weird. I bottled my apple cider a few weeks ago and made it still. I did not put sugar in the cider to carbonate it. I put potassium metabisulphate in the carboyy about a week after I took my final gravity reading. Let it sit for a little more than a week, then bottled it. The potassium should have killed all of the yeast in the cider.

Tonight, I opened a bottle to try it and it hissed at me and even smoked. This thing is VERY carbonated! It is acting almost like a champaign. It has been in my glass for about 15 minutes now and it looks like I dropped an alka seltzer in it. Lots of rising bubbles.

How did this happen? It isnt a bad thing for me, per se, because I LIKE carbonated ciders, but I have no clue how this happened.

Any thoughts?

Possible infection? Any small piece of debris in the bottle can cause an infection, don't ask how I know.

hmmm...tastes fine, though. Just weird.

Still no thoughts on this? Could the potassium metabisulfate not have killed the yeast causing it to carbonate after I put in the apple juice concentrate for back sweetening? I cant think of anything else that could have carbonated it. I dont think it was an infection at all.