Rockslides at high elevations often transform into rock avalanches due to fragmentation, posing a significant threat. However, the underlying mechanism for the high mobility of rock avalanches remains unclear. The discrete element method is employed to simulate the process of blocks varying in rock strength moving on an inclined plane varying in slope angle, impacting the horizontal plane and subsequently spreading. The internal damage distribution, velocity profile, granular agitation, and energy conversion are analyzed. The results indicate multistyle fragmentation modes, including sliding friction fragmentation, compressive collision fragmentation, and bending tensile fragmentation. Rock strength and slope angle influence the final fragmentation phenomenon and degree of fragmentation by altering the weights of different fragmentation modes. A non-monotonic and segmented relationship between the degree of fragmentation and friction coefficient is found, which appears to be induced by the competitive relationship between positive and negative feedback effects on mobility. When only impact fragmentation occurs, despite the boost in horizontal momentum facilitating the transport of fragments, the negative feedback effect on mobility caused by impact fragmentation energy consumption plays a dominant role. Basal fragmentation occurs under specific combinations of rock strength and slope angle, causing a rock avalanche in a shear-dominated dense flow state with low internal disturbance. At this point, the positive feedback effect on mobility caused by the basal fragmentation-induced unique flow structure takes the lead. This study highlights the contribution of flow regime changes induced by fragmentation to energy conversion, thereby affecting the mobility of rock avalanches.