  We theoretically investigate multinucleon transfer (MNT) processes in
$^{238}$U+$^{124}$Sn reaction at $E_\mathrm{lab}=5.7$ MeV/$A$ using the
time-dependent Hartree-Fock (TDHF) theory. For this reaction, measurements of
MNT processes have been reported, showing substantial MNT cross sections
accompanying more than ten protons. From the calculation, we find that the
amount of transferred nucleons depends much on the relative orientation between
the deformation axis of $^{238}$U and the relative vector connecting centers of
$^{238}$U and $^{124}$Sn nuclei. We find a formation of thick neck when the
$^{238}$U collides from its tip with $^{124}$Sn. However, the neck formation is
substantially suppressed when $^{238}$U collides from its side. We have found
that a large number of protons are transferred in the tip collision. This is
caused by the breaking of the neck and subsequent absorption of nucleons in the
neck region. We thus conclude that the measured MNT processes involving about
ten protons originate from the neck breaking dynamics in the tip collisions of
a deformed $^{238}$U nucleus.
