The most likely explanation is aircraft lights, not an astronomical flare.

From the community answers, the longest streak looks consistent with a low-resolution airplane track: in a 15-second exposure, an aircraft’s red navigation or anti-collision lights can record as a bright red line, especially when the plane is low on the horizon. If thrust, haze, or atmospheric distortion is involved, the light can look broader or more intense.

The shorter streaks could also be small meteors, which commonly appear as short tapered streaks in long exposures.

So the best interpretation is:

• longest/brightest red streak: probably an aircraft light trail
• shorter streaks: possibly small meteors

Because you saw the event visually, it was a real external light source rather than sensor flare or internal lens reflections. The low elevation above the horizon also favors aircraft, since planes are commonly seen there and atmospheric scattering can enhance red light.