In the paper https://arxiv.org/abs/1712.00457 about rotation rates of gas giants it says:
"owing to accumulation of angular momentum stored in the source material, a planetary mass object should rotate at or near breakup towards the end of the phase of rapid of gas accretion (irrespective of whether the object formed through core accretion or via gravitational instability). In light of this expectation and the discussion presented above, another mechanism is needed to reduce the rotation rate to values well below break-up, and counteract spin-up due to gravitational contraction and accretion. Because our observations do not show a statistically significant dependence of angular velocities on age, we speculate that the spin-down process (whatever it may be) operates exclusively during the disk-bearing stage of evolution."
Question: What mechanisms would cause the rotation of a gas giant to slow down, assumming that the planet is far enough from its star that tidal interaction with the star won't slow it down?