  In recent years, more and more gamma-ray bursts with late rebrightenings in
multi-band afterglows unveil the late-time activities of the central engines.
GRB 100814A is a special one among the well-sampled events, with complex
temporal and spectral evolution. The single power-law shallow decay index of
the optical light curve observed by GROND between 640 s and 10 ks is
$\alpha_{\rm opt} = 0.57 \pm 0.02$, which apparently conflicts with the simple
external shock model expectation. Especially, there is a remarkable
rebrightening in the optical to near infrared bands at late time, challenging
the external shock model with synchrotron emission coming from the interaction
of the blast wave with the surrounding interstellar medium. In this paper, we
invoke a magnetar with spin evolution to explain the complex multi-band
afterglow emission of GRB 100814A. The initial shallow decay phase in optical
bands and the plateau in X-ray can be explained as due to energy injection from
a spin-down magnetar. At late time, with the falling of materials from the
fall-back disk onto the central object of the burster, angular momentum of the
accreted materials is transferred to the magnetar, which leads to a spin-up
process. As a result, the magnetic dipole radiation luminosity will increase,
resulting in the significant rebrightening of the optical afterglow. It is
shown that the observed multi-band afterglow emission can be well reproduced by
the model.
