  Recent high-precision mass measurements of $^{9}$Li and $^{9}$Be, performed
with the TITAN Penning trap at the TRIUMF ISAC facility, are analyzed in light
of state-of-the-art shell model calculations. We find an explanation for the
anomalous Isobaric Mass Multiplet Equation (IMME) behaviour for the two $A$ = 9
quartets. The presence of a cubic $d$ = 6.3(17) keV term for the $J^{\pi}$ =
3/2$^{-}$ quartet and the vanishing cubic term for the excited $J^{\pi}$ =
1/2$^{-}$ multiplet depend upon the presence of a nearby $T$ = 1/2 state in
$^{9}$B and $^{9}$Be that induces isospin mixing. This is contrary to previous
hypotheses involving purely Coulomb and charge-dependent effects. $T$ = 1/2
states have been observed near the calculated energy, above the $T$ = 3/2
state. However an experimental confirmation of their $J^{\pi}$ is needed.
