What is the primary reason that icy objects in the Kuiper Belt have highly elliptical and tilted orbits?

Icy Kuiper‑Belt objects ended up on tilted, elongated paths because the giant planets, especially Neptune, tugged on them while the planets were still moving and settling. As Neptune migrated outward, it gravitationally scattered nearby planetesimals, giving them higher eccentricities and inclinations. The repeated close passes and resonances pushed many objects into more tilted and elongated orbits, much like a ball that has been repeatedly kicked in different directions. For example, a small icy body that started in a circular orbit can be nudged by Neptune’s gravity into a long, slanted orbit, similar to how a child’s toy car can be thrown off its straight track by a sudden push. Thus, Neptune’s migration and gravitational scattering are the main reasons for the Kuiper Belt’s chaotic, tilted orbits.