On 6 July 2020, 3 h 40 min after rainfall stopped, a delayed debris-flow disaster occurred due to colluvium deposits in a hollow region (CDH) in the Chenghuangmiao gully, Sichuan Province, China, which resulted in 4 deaths and 27 injuries. This study explores the initiation process of the delayed debris flow and the cause for the delay. Field investigations, catchment geometry interpretation, laboratory tests, theoretical calculations, and fluid-solid coupling numerical simulation were performed to obtain landslide parameters and understand the mechanisms of the event. Results show: (1) The event was a giant low-frequency viscous debris flow. (2) Its initiation was caused by the delayed landslide process under the influence of back-end confluence. (3) The debris-flow discharge in the main gully increased over 19.5 min. (4) The seepage process inside the CDH continued for 3 h 20 min after the rainfall stopped before its pore pressure and reduction in strength was sufficient to initiate the debris flow. This research provides new insights on delayed debris-flow disasters; it is a reference for improving disaster management systems, especially monitoring and early warning systems, thereby avoiding future casualties.