The 2018 MW 7.5 Palu-Donggala, Indonesia earthquake initiated a series of catastrophic flowslides that claimed the lives of several thousand people, making it the deadliest natural disaster of 2018. Three of the most significant flowslides, which occurred in the communities of Petobo, Jono Oge, and Lolu Village, were clustered within alluvial sediments of the Palu River Valley. Post-earthquake field reconnaissance, subsurface investigation, eyewitness interviews, and geomechanical analyses indicate that the failure sequence began when the earthquake ground motion raised pore water pressures in the saturated interbedded alluvial sediments. The flowslides were not coseismic but instead occurred several minutes after earthquake shaking ended. We hypothesize that high excess pore pressures created large hydraulic gradients within the alluvial sediments, which ultimately triggered progressive liquefaction that led to a redistribution of stresses and accumulation of shear strains. The flowslides were then initiated as gravitational driving stresses began to exceed the residual strength of the liquefied soil. Given the interbedded stratigraphy, we speculate that void redistribution and the development of water films possibly contributed to low residual strengths. An adjacent unlined agricultural canal played a critical role in the failure sequence by artificially raising the groundwater level, thereby increasing the susceptibility to liquefaction and attendant strength loss.