  The recent experimental observation of isospin symmetry breaking (ISB) in the
ground states of the $T=3/2$ mirror pair $^{73}$Sr - $^{73}$Br is theoretically
studied using large-scale shell model calculations. The large valence space and
the successful PFSDG-U effective interaction used for the nuclear part of the
problem capture possible structural changes and provide a robust basis to treat
the ISB effects of both electromagnetic and non-electromagnetic origin. The
calculated shifts and mirror-energy-differences are consistent with the
inversion of the $I^{\pi}$= 1/2$^{-}, 5/2^{-}$ states between $^{73}$Sr -
$^{73}$Br, and suggest that the role played by the Coulomb interaction is
dominant. An isospin breaking contribution of nuclear origin is estimated to be
$\approx 25$ keV.
