bottles carbonating after back sweetening

Hi All, First time mead maker, long time beer brewer. Bottles carbonating after a couple of months in storage, unclear on reason because I sanitized religiously, campden tableted the brew as part of the back sweetening and confirmed that there was no fermentation for several weeks after. 7 bottles in total, a couple are confirmed fine, others gush a bit when opened and the taste is a little tangy in comparison.

As a beer brewer the only thing I can think is that some of the bottles have picked up a wild yeast? What do you reckon and what should I do with the carbed bottles, can they be saved? Thanks!