  Recent experiments have demonstrated the generation of widely-spaced
parametric sidebands that can evolve into "clustered" optical frequency combs
in Kerr microresonators. Here we describe the physics that underpins the
formation of such clustered comb states. In particular, we show that the
phase-matching required for the initial sideband generation is such that (at
least) one of the sidebands experiences anomalous dispersion, enabling that
sideband to drive frequency comb formation via degenerate and non-degenerate
four-wave mixing. We validate our proposal through a combination of
experimental observations made in a magnesium-fluoride microresonator and
corresponding numerical simulations. We also investigate the coherence
properties of the resulting clustered frequency combs. Our findings provide
valuable insights on the generation and dynamics of widely-spaced parametric
sidebands and clustered frequency combs in Kerr microresonators.
