There are so many possibilities as to what this could be. Fire is the only thing that can kill wights , so perhaps this was done on purpose by the Starks to save the family — but that's probably an optimistic guess. It could be the result of a war tactic from Cersei, who, as we saw in season 6, has no problem putting entire cities up in flames. Or, they're doing what WOTW says they did in season two and six for the wildfire, which is use regular fire for filming that they then change in editing to be the blue "flame" of Viserion . No matter how you slice it, Winterfell is toast.