A deep-seated flexural toppling failure of rock masses is located on the right bank upstream of the Lancang River, Southwest China. The rock masses unloaded intensely in two directions and formed intense toppled rock masses with lengths and widths of 1200 m and 950 m, respectively, and volumes over 1.5 × 10⁸ m³. The toppled rock masses could be classified into three grades from surface to deep rock masses: strongly, moderately and weakly toppled rock masses. The strong unloading of rock masses during the rapid downcutting process of rivers was one of the main external factors that induced deep-seated toppling failure. The mechanism could be summarized as follows: intense unloading destroyed the structure of rock masses and weakened the mechanical properties of rock masses by forming many unloading cracks in rock masses; additionally, the stress field of the slope was further redistributed and a stress relaxation zone formed in the strong unloading zone. Then, the stresses of rock masses in the relaxation zone were dominated by gravity, and the deep-seated toppling failure formed in the strongly unloaded rock masses under the action of gravity.