  Axisymmetric disks of eccentric orbits in near-Keplerian potentials are
unstable to an out-of-plane buckling. Recently, Zderic et al. (2020) showed
that an idealized disk saturates to a lopsided mode. Here we show that this
apsidal clustering also occurs in a primordial scattered disk in the outer
solar system which includes the orbit-averaged gravitational influence of the
giant planets. We explain the dynamics using Lynden-Bell (1979)'s mechanism for
bar formation in galaxies. We also show surface density and line of sight
velocity plots at different times during the instability, highlighting the
formation of concentric circles and spiral arms in velocity space.
