  Dwarf satellites in galaxy groups are distributed in an anisotropic and
asymmetric manner, which is called the ``lopsided satellite distribution''.
This lopsided signal has been observed not only in galaxy pairs but also in
isolated systems. However, the physical origin of the lopsided signal in
isolated systems is still unknown. In this work, we investigate this in the
state-of-the-art hydrodynamical simulation of the MillenniumTNG Project by
tracing each system back to high redshift. We find that the lopsided signal is
dominated by satellites located in the outer regions of the halo and is also
dominated by recently accreted satellites. The lopsided signal originates from
the anisotropic accretion of galaxies from the surrounding large-scale
structure and that, after accretion, the nonlinear evolution of satellites
inside the dark-matter halo weakens the lopsidedness. The signal decreases as
cosmic time passes because of a competition between anisotropic accretion and
internal evolution within dark matter halos. Our findings provide a useful
perspective for the study of galaxy evolution, especially for the origin of the
spatial satellite galaxy distributions.
