Cgs swing comes from converting the depletion region to
inversion, now you "hook up" the area under the gate
(Cox) to the source via the channel. There is only so
much charge to be put, for a fixed Vgs, and it all happens
before to shortly above threshold.
Then the drain begins to swing, and that high drain
drain voltage across drain-gate capacitances is the
plateau charge. Vgate/RgateDrive = I = Cds*dV/dt,
generally the fixed gate drive creates a linear ramp
on the FET drain, and this is why low output impedance
gate drivers are needed for high frequency & efficiency.