  Restoration of pseudo-spin symmetry (PSS) along the $N=32$ and $34$ isotonic
chains and the physics behind are studied by applying the relativistic
Hartree-Fock theory with effective Lagrangian PKA1. Taking the proton
pseudo-spin partners $(\pi2s_{1/2},\pi1d_{3/2})$ as candidates, systematic
restoration of PSS along both isotonic chains is found from sulphur (S) to
nickel (Ni), while distinct violation from silicon (Si) to sulphur is
discovered near the drip lines. The effects of the tensor-force components
introduced naturally by the Fock terms are investigated, which can only partly
interpret the systematics from calcium to nickel, but fail for the overall
trends. Further analysis following the Schr\"{o}dinger-like equation of the
lower component of Dirac spinor shows that the contributions from the Hartree
terms dominate the overall systematics of the PSS restoration, and such effects
can be self-consistently interpreted by the evolution of the proton central
density profiles along both isotonic chains. Specifically the distinct PSS
violation is found to tightly relate with the dramatic changes from the
bubble-like density profiles in silicon to the central-bumped ones in sulphur.
