The Golden Company probably employed the same tactic as the Boltons did to the Stark loyalists outside of Winterfell. The only difference being that Theon and the ironborn knew it was happening. Entirely probable that the Golden Company raises its banners, surprises and crushes the Tyrell holding force and claims service to Stannis. Stannis garrison opens the gates and BAM! take the keep unawares. Obviously the first suspiscion should be treachery but Stannis is stuck in the North and snowed in, probably unable to get Ravens out to Storm's End anyway. I can't remember right now but who is the garrison commander at Storm's End? Is he ever mentioned?
