Two integrated core / scrape-off-layer / divertor transport codes TOPICS-IB and JINTRAC with links to MHD stability codes have been coupled with models of pellet injection to clarify effects of pellet on the behavior of Edge Localized Modes (ELMs). Both codes predicted the following two triggering mechanisms. The energy absorption by the pellet and its further displacement due to the ExB drift, as well as transport enhancement by the pellet, were 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 ExB drift. Further, the sharp increase in local density and temperature gradients in the vicinity of ablated cloud could cause 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.