Two integrated core/scrape-off layer/divertor transport codes TOPICS-IB and JINTRAC with links to MHD stability codes are coupled with models of pellet injection to clarify effects of the pellet on the behaviour of edge-localized modes (ELMs). Both codes predict the following two triggering mechanisms. Energy absorption by the pellet and its further displacement due to the E × B drift, as well as transport enhancement by the pellet, are found to be able to trigger the ELM. The ablated cloud of pellet absorbs the background plasma energy and causes a radial redistribution of pressure due to the subsequent E × B drift. Further, the sharp increase in local density and temperature gradients in the vicinity of ablated cloud causes the transient enhancement of heat and particle transport. Both mechanisms produce a region of increased pressure gradient in the background plasma profile within the pedestal, which triggers the ELM. The mechanisms have the potential to explain a wide range of experimental observations.