Using first-principles-based Hamiltonian, we show novel structural behaviors of hexagonal manganite ${\mathrm{BaMnO}}_{3}$ at moderate low temperatures ($T\mathrm{s}$). The structural change of ${\mathrm{BaMnO}}_{3}$ occurring at $\ensuremath{\sim}130$ K is assigned to an improper transition between the low-$T$ ferroelectric phase with $P{6}_{3}cm$ symmetry and a high-$T$ nonpolar phase with $P{6}_{3}/mmc$ symmetry, induced by coupled softening of the ${K}_{3}$ and ${\mathrm{\ensuremath{\Gamma}}}_{2}^{\ensuremath{-}}$ modes. Herein, the order parameters show unconventional behaviors, with the mode amplitude being order-disorder like and the mode phase being displacive. Optical Goldstone-like mode, implied by the Mexican-hat-like Born-Oppenheimer potential energy surface, is manifested in our simulations via tuning quantum fluctuations and lattice strains. As this mode can be enhanced by lowering the contribution of intersite interactions, we suggest an experimental detecting scheme by inducing a light active $\mathrm{\ensuremath{\Gamma}}$-point mode strongly coupled to it.