Not sure but it could be that materials like other similar materials. Water dense materials will group, carbon dense and rock dense will also group. Also density might group heavier things in the center (like the planets).
Edit from NASA’s website:
“Spiral Density Waves
In spiral density waves, the gravitional pull of Saturn and its moons causes the ring particles to bunch together or spread apart in a tightly-wound spiral pattern. The result is that we can see a series of “bands” in certain parts of the rings. You can tell density waves from the other type (bending waves) by the way the distance between the bands (“wavelength”) changes. In density waves, the wavelength of the bands DECREASES as you get farther from Saturn. In other words, the bands get squeezed closer together as you get farther from Saturn.”