The mechanism responsible for the lambda-phase transitions in ammonium halides is investigated within the framework of an extended theory of the disorder-induced Raman scattering. The Raman spectra of ammonium halides have been predicted for both the disordered beta phase (NH4Cl and NH4Br) and the ordered gamma phase (NH4Br). It is postulated that those terms which dominate the dynamic distortions of the lattices and thereby cause the bulk of the disorder-allowed Raman scattering, will also dominate the quasi-static distortion which is the lambda-phase transition in these crystals. From this we find that the four-spin correlation function for the M-point boundary phonons in the disordered phase (beta), and the two-spin correlation function for the zone centre phonons in the ordered phases (gamma and delta) dominate the mechanism of the lambda-phase transitions in ammonium halides. Predictions obtained for the Raman scattering agree in most cases with our Raman observations on NH4Cl and NH4Br.