In the Shangzhai dialect of Horpa, an under-studied Tibeto-Burman language of northwestern Sichuan, pervasive phonological alternations occur in the morphological causative formation. This paper applies the study of this phenomenon to the analysis of the historical development of alternative modes of encoding causativity in Horpa and two related rGyalrongic languages: rGyalrong (proper) and Lavrung. Despite bewildering surface variations, Shangzhai Horpa can be analyzed as having a single consistently non-syllabic causative prefix s- which exerts pressure on the already elaborate onset system and triggers multiple phonological adjustments. The excessive allomorphy and constraints exhaust the morphological means of causation coding, leading to the rise of the periphrastic causative construction as the primary causativizing strategy in the language. By contrast, the dominant mode of expressing causativity still rests in the realm of derivational morphology in the other rGyalrongic languages where the old causative prefix *s˙- remains syllabic.