  Axisymmetric disks of eccentric Kepler orbits are vulnerable to an
instability which causes orbits to exponentially grow in inclination, decrease
in eccentricity, and cluster in their angle of pericenter. Geometrically, the
disk expands to a cone shape which is asymmetric about the mid-plane. In this
paper, we describe how secular gravitational torques between individual orbits
drive this "inclination instability". We derive growth timescales for a simple
two-orbit model using a Gauss $N$-ring code, and generalize our result to
larger $N$ systems with $N$-body simulations. We find that two-body relaxation
slows the growth of the instability at low $N$ and that angular phase coverage
of orbits in the disk is important at higher $N$. As $N \to \infty$, the
e-folding timescale converges to that expected from secular theory.
