  Metal insulator transition with C-type orbital ordering (OO) is generic among
RMn$^{3+}$O$_3$(R=rare earth) perovskites with a $Pbnm$ ground state.
Distinctly, TlMnO$_3$ shows a very different rocksalt (G-type) OO together with
the emergence of an unusual triclinic $P\overline{1}$ structure. Employing
first principles calculations, and symmetry mode analysis we investigated
structural and electronic origin of G-type OO in TlMnO$_3$. We reveal that
Jahn-Teller (JT) distortion in TlMnO$_3$ is structurally triggered,
consequently giving rise to G type OO. Similar mechanism is rather common in
RNi$^{3+}$O$_3$ perovskites, where breathing of NiO$_6$ is observed. We further
reveal that, triggering of breathing distortion is not limited to nickelates
but also present in TlMnO$_3$ (and LaMnO$_3$). In fact, other types of JT
distortion, are also show triggering mechanism, although final ground state of
these systems result from subtle anharmonic interactions.
