The phase transition sequence of SrBi2Ta2O9 (SBT) and the local microscopic dynamics near the ferroelectric transition are investigated using a shell model with parameters fitted to first-principle calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions. In this way the existence of an intermediate orthorhombic paraelectric phase, characterized by the rotation of the TaO6 octahedra, is demonstrated without using any explicit experimental data as input. The local polarization dynamics does not provide any indication of a relaxation process near the ferroelectric transition. Finally, dielectric and piezoelectric coefficients along crystallographic directions are investigated.