Why does a spiral retaining ring 'Pop Out' during high-speed rotation even without an axial load?

This is almost always due to the centrifugal force exceeding the ring's 'Grip on the Groove'. As the assembly rotates, the mass of the ring generates a radial force $F_c = m r \omega^2$ that acts to expand an external ring (or contract an internal one). If this force overcomes the initial elastic preload (clinging force) of the ring, it will lift off the groove bottom. Once lifted, any slight vibration or axial force will cause the ring to eject. The solution is to use a 'Self-Locking' ring or to increase the material's thickness to increase the radial stiffness and the initial seating force.