  A new component was reported in the X-ray counterpart to the binary
neutron-star merger and gravitational wave event GW170817, exceeding the
afterglow emission from an off-axis structured jet. The afterglow emission from
the kilonova/macronova ejecta may explain the X-ray excess but exceeds the
radio observations if the spectrum is the same. We propose a fallback accretion
model that a part of ejecta from the neutron star merger falls back and forms a
disk around the central compact object. In the super-Eddington accretion phase,
the X-ray luminosity stays near the Eddington limit of a few solar masses and
the radio is weak, as observed. This will be followed by a power law decay. The
duration of the constant luminosity phase conveys the initial fallback
timescale $t_0$ in the past. The current multi-year duration requires $t_0 >
3$--$30$ sec, suggesting that the disk wind rather than the dynamical ejecta
falls back after the jet launch. Future observations in the next decades will
probe the timescale of $t_0 \sim 10$--$10^4$ sec, around the time of extended
emission in short gamma-ray bursts. The fallback accretion has not been halted
by the $r$-process heating, implying that fission is weak on the year scale. We
predict that the X-ray counterpart will disappear in a few decades due to the
$r$-process halting or the depletion of fallback matter.
