Steve is right. It's going to be one of those two explanations. The good news is that those are disparate explanations and you can review your processes to figure out which is the likely culprit.
I am far from a cider expert but I'd think about hitting it with campden again and see if you can kill off whatever is active there. With a sub-1000 gravity I'd guess it is wild yeast rather than bacteria because you're out of sugars for bacteria to consume. Wild yeast, and particularly brett, can metabolize alcohol, esters, etc. that are currently available food sources. Campden won't kill off brett and some other yeast but it will prevent them from budding and multiplying, which will at least slow the effects of the infection.