Why do Pluto’s smaller moons spin rapidly and chaotically?

The smaller moons of Pluto—Hydra, Nix, Kerberos, and Styx—spin rapidly and chaotically primarily because of the ever-changing gravitational field created by the interaction between Pluto and its largest moon, Charon, as they rotate around each other.1

Unlike most satellites in the solar system, which are in synchronous rotation (where their rotation period matches their orbital period), these four smaller moons are not synchronized with the planet. This lack of synchronization, combined with the complex gravitational environment, causes the moons to tumble in an unpredictable manner, with some, such as Nix and Hydra, occasionally experiencing pole-flipping.1

While these moons maintain stable dynamic resonances with Charon and each other—meaning they pass one another in predictable, periodic intervals—the gravitational "shoves" they receive from the rotating Pluto-Charon system prevent them from settling into a stable, synchronous spin.1