We report the results of a comprehensive study of the critical dynamics of the classical perovskite antiferroelectric PbZrO3. The combination of inelastic x-ray and diffuse x-ray scattering techniques and Brillouin light scattering was used. It is found that the dispersion of the transverse acoustic (TA) phonons is strongly anisotropic. The dispersion curve of the in-plane polarized TA phonons propagating in [1 1 0] direction demonstrates pronounced softening. Slowing down of the excitations at R point is found, it is manifested in growing of the central peak. This slowing down is too weak to be considered as a primary origin of the corresponding order parameter. Obtained results are treated in terms of TA-transverse optic (TO) flexoelectric mode coupling. It is demonstrated that the structural phase transformation in PbZrO3 can be considered as the result of the only intrinsic instability associated with the ferroelectric soft mode.