  The simplicity in the nuclear quadrupole moments reported recently in
$^{107-129}$Cd, i.e., a linear increase of the ${11/2}^-$ quadrupole moments,
is investigated microscopically with the covariant density functional theory.
Using the newly developed functional PC-PK1, the quadrupole moments as well as
their linear increase tendency with the neutron number are excellently
reproduced without any {\it ad hoc} parameters. The core polarization is found
to be very important and contributes almost half of the quadrupole moments. The
simplicity of the linear increase is revealed to be due to the pairing
correlation which smears out the abrupt changes induced by the single-particle
shell structure.
