  Disks of low-mass bodies scattered by giant planets to large semi-major axis
and constant periapsis orbits are vulnerable to a buckling instability. This
instability exponentially grows orbital inclinations, raises periapsis
distances, and coherently tilts orbits resulting in clustering of arguments of
periapsis. The dynamically hot system is then susceptible to the formation of a
lopsided mode. Here we show that the timescale of the buckling instability
decreases as the radial surface density of the population becomes more
centrally dense, i.e., steeper scattered disks buckle faster. Accounting for
differential apsidal precession driven by giant planets, we find that
$\sim\!10\,M_\oplus$ is sufficient for a primordial scattered disk in the
trans-Neptunian region to have been unstable if $dN \propto a^{-2.5} da$.
