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Why does Nereid have such an unusually stretched orbit around Neptune?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Nereid’s highly elliptical and unusually stretched orbit is believed to be a consequence of the gravitational disruption caused by the capture of Triton into the Neptunian system.12

When Triton was captured by Neptune, its own orbit was initially highly eccentric. As tidal interactions between Triton and Neptune gradually circularized Triton's path, this "settling-in" process severely disturbed the planet's original system of moons. It is hypothesized that this chaotic period resulted in the ejection of other moons, collisions, or the significant alteration of existing orbital paths, leaving Nereid with its current, highly eccentric orbit. Alternatively, it remains possible that Nereid itself is a captured object that formed elsewhere in the solar system.12