  Recently reported anomalies in frequency shift of order $\delta f\sim 0.1
-10\,\mbox{kHz}$ for Niobium SRF cavities in a narrow temperature region near
$T_c\simeq 9\,\mbox{K}$ are sensitive to the surface preparation and Nitrogen
doping. We developed methods for calculating the surface current response of Nb
SRF cavities, as well as the resonant frequency shift and quality factor, as
functions of temperature, frequency and disorder based on the Keldysh
formulation of the theory for nonequilibrium superconductivity coupled with
Maxwell's theory and boundary conditions for the response functions and
electromagnetic field. We show that the anomaly is sensitive to impurity
disorder. Our results for the anomaly in the frequency shift are in good
agreement with experimental results for Nb with an anisotropic gap on the Fermi
surface and inhomogeneous non-magnetic disorder. We also show that the quality
factor as a function of the impurity scattering rate is maximum for in SRF
cavities with intermediate disorder, $\hbar/\tau\Delta\sim{\cal O}(1)$ with the
maximum $Q$ decreasing with increasing frequency.
