Comment by mapt

Even in a purely planar distribution, nodal precession still occurs slowly.

It doesn't even need to be factored in, though, if different planes are colliding with each other and energetically generating a spectrum of new orbital vectors (many less than circular) from impact. This effect colludes with altitude drop from orbital decay and the tendency to circularize orbits by perigee drag, to make it so that higher orbit debris percolate into lower orbits over time.