This is most likely lens flare. A bright light source—probably the sun just behind the cloud—hit the GoPro’s lens at an angle and created a horizontal flare across the frame.

Flare can appear only in a single shot if the camera angle changes slightly, which fits your note that the camera was moving quickly. Different lens designs produce different flare shapes, and some wide-angle lenses can create long streaks like this.

Because it’s an optical artifact rather than a sensor problem, it’s normal that it wouldn’t show up in the shots before or after.