Cider may be starting back up?

I have a question on my five gallons of cider. Started it about a month ago and all went well. Started at 1.06 with 1968 wyeast. Finished, or I stopped fermentation at 1.0 Racked it into a carboy and then I added 1/4 tsp potassium metabisulfite and 1 1/4 tsp potassium sorbate. Back sweetened then with three cans of apple juice concentrate. All seemed well and I was just bulk aging it in the carboy but today it looked like it was starting back up? Bubbles in the top of the cider! So I put the carboy in the fridge to cold crash it. Is this normal? Should I have put this in bottles already? Its still dark so I figured I needed to let it sit a bit before I bottle. Just for clarification, its not murky just dark in color. Any thoughts?

Definitely sounds like it is fermenting again. It's a good thing it wasn't bottled- you'd have bottle bombs!

When fermentation is finished, and the cider is clear, it's racked off of the lees and into the sorbate/sulfite mixture. Usually it works, but not always. Sorbate doesn't stop fermentation, but inhibits yeast reproduction. It sounds like you had enough yeast to ferment out the juice you used to sweeten. Or, maybe the sorbate was old.

In any case, you could wait until it's completely done, and try again.