A first-principles-based scheme is used to investigate the nature of the ferroelectric phase transition in the multiferroic BiFeO3. This transition possesses several order parameters (that are components of the polarization and of the tilting of oxygen octahedra) with none of them being primary and is solely driven by the collaborative coupling between the polarization and the other order parameters. This phase transition is therefore neither proper nor improper but rather can be regarded as a special trigger-type transition. Our atomistic simulations also reveal the precise origins and mechanisms of this trigger-type phase transition. © 2009 The American Physical Society.