It's most likely an infection from not properly cleaning something/everything that touches the wort/beer. Then when you sanitize,if you did sanitize everything,the dirty stuff could harbor a few nasties that survived. Or you just didn't remember to saitize something that touched the beer. Like racking tubes & such. Or if your fermenters have spigots like your bottling bucket,& you didn't remove & clean them & the mounting hole. That sort of thing.