Intense rainfall can produce rapid inflows that overtop or erode landslide dams, triggering cascading failures that entrain large volumes of sediment. This process rapidly transforms the floods into dense debris flows, whose size and destructive potential grow as they travel downstream. Therefore, understanding the mechanisms of cascading failure is crucial for flood risk assessment and disaster mitigation in mountainous areas. This study uses flume experiments and numerical simulations to investigate the transition of rapid floods into debris flows during the cascading failure of four landslide dams. Results show that successive dam breaches increase sediment entrainment, transforming the initial water flow into a dense debris flow. Numerical simulations incorporating erosion and entrainment reproduce the experimental results and reveal that hydrodynamic parameters-velocity, shear stress, and discharge-can increase by up to threefold after successive dam breaches. This amplification persists even when the upstream reservoir volume is low, sustained by a feedback mechanism in which higher velocity increases shear stress, which accelerates erosion and sediment entrainment. The resulting rise in flow density further enhances shear stress, creating a cycle that amplifies discharge. Conversely, when sediment entrainment is neglected, this positive feedback loop is suppressed, leading to a significant reduction in the amplification effect. These findings enhance our understanding of scale amplification by cascading failure, and provide a scientific basis for debris flow mitigation strategies in mountainous areas.