Low-frequency heavy rainfall events commonly trigger debris flows. However, it is not currently understood how high-frequency, light rainfall can trigger giant catastrophic debris flow events with return periods of greater than 100 years; for example, the 2014 event in the Erkazi gully (EKZG), Danba County, Sichuan Province, China. This study uses the EKZG debris flow as a case study to determine the main factors controlling the formation of low-frequency debris flows and their generation by high-frequency light rainfall through field investigations, indoor and outdoor experiments, simulated calculations, and statistical analysis. We revealed that the soil mass predominantly controlled the generation of the EKZG debris flow. Moreover, internal geological factors such as earthquakes and external geological factors such as dry-wet cycles and freeze-thaw erosion likely dominated the EKZG debris flow formation process. In contrast, the geomorphological characteristics, basaltic columnar jointing, and earthquakes jointly determined the amplification process of the debris flow. Furthermore, we discussed the critical role of community-based warning systems during debris flow disasters. For example, early identification of the blocking characteristics of the EKZG debris flow by observers enabled the successful real-time evacuation of 651 people. The following recommendations are proposed: (1) future research should focus on the dynamic replenishment process of the soil mass through dry-wet and freeze-thaw cycles, and (2) the effectiveness and applicability of community-based warning systems should be improved for the prevention and mitigation of geological hazards.