  Using high-resolution {\sc Romulus} simulations, we explore the origin and
evolution of the circumgalactic medium (CGM) in the region 0.1 $\leq
\mathrm{R}/\mathrm{R}_\mathrm{500} \leq$ 1 around massive central galaxies in
group-scale halos. We find that the CGM is multiphase and highly dynamic.
Investigating the dynamics, we identify seven patterns of evolution. We show
that these are robust and detected consistently across various conditions. The
gas cools via two pathways: (1) filamentary cooling inflows and (2)
condensations forming from rapidly cooling density perturbations. In our
cosmological simulations, the perturbations are mainly seeded by orbiting
substructures. The condensations can form even when the median $t_\mathrm{cool}
/ t_\mathrm{ff}$ of the X-ray emitting gas is above 10 or 20. Strong amplitude
perturbations can provoke runaway cooling regardless of the state of the
background gas. We also find perturbations whose local $t_\mathrm{cool} /
t_\mathrm{ff}$ ratios drop below the threshold but which do not condense.
Rather, the ratios fall to some minimum value and then bounce. These are weak
perturbations that are temporarily swept up in satellite wakes and carried to
larger radii. Their $t_\mathrm{cool} / t_\mathrm{ff}$ ratios decrease because
$t_\mathrm{ff}$ is increasing, not because $t_\mathrm{cool}$ is decreasing. For
structures forming hierarchically, our study highlights the challenge of using
a simple threshold argument to infer the CGM's evolution. It also highlights
that the median hot gas properties are suboptimal determinants of the CGM's
state and dynamics. Realistic CGM models must incorporate the impact of mergers
and orbiting satellites, along with the CGM's heating and cooling cycles.
