It is widely believed that switching to the conductive state in memristive materials is triggered by the external field that drives defect dynamics. In polycrystalline materials, grain boundaries are further believed to cause switching by enabling faster defect motion. Here, we report a first-principle study of oxygen vacancy dynamics at a grain boundary (GB) in polycrystalline ZnO and show that switching to the conductive state is triggered by a recombination-enhanced motion of vacancies perpendicular to the GB. We call this mechanism the “breathing” trigger of memristive switching.