  We present results for the resonant frequency shift and quality factor of
disordered Nb SRF cavities driven out of equilibrium by the resonant microwave
field. The theory is based on the nonequilibrium theory of superconductivity
for the current response to the electromagnetic field at the vacuum-metal
interface. We are able to accurately predict the observed frequency shifts with
a precision of order several Hz over the full temperature range $0 < T \le
T_c$, including the negative frequency shift anomalies that are observed very
near $T_c$. The origin of these anomalies is shown to be the competition
between the normal metal skin depth and the London penetration depth which
diverges as $T\rightarrow T_c^-$. An analytic approximation to the full current
response, valid for $|T-T_c|\ll T_c$, accounts for the negative frequency shift
near $T_c$. The non-monotonic dependence of the quality factor on the
quasiparticle scattering rate is related to the pair-breaking effect of
disorder on the superfluid fraction, and thus the London penetration depth.
