A dry rub is only dry when its applied (since moisture comes out of the meat), so I don't think it is due to excess moisture. And if the rub is as advertised it ought to kill the fungus spores wet or dry.
I'd be surprised if the brisket would get fuzzy if just left in the fridge with no rub applied for that length of time. To me it suggests that the rub is somehow contaminated and inactive too. Or the bag or something.