As a sedimentary marker of high-energy fluid transport, the mudball formation mechanism is of great significance to the erosion process during extreme climate events. This study uses the extreme rainfall event in Dingbian County of the Loess Plateau in 2022 as a research context. Through integrated approaches including drone imagery and in situ measurements, we systematically investigated the sedimentary characteristics and dynamic processes of mudballs in several small watersheds, focusing on the three-phase mechanism of gravitational erosion, such as mudflow transport and dynamic sorting. The results show that mudball formation is constrained by the dual thresholds of physical source supply and hydraulic conveyance. The proportionate area of gravitational erosion should be >= 5 parts per thousand, and the watershed elevation difference and gully length ratio (Delta H/L1) should range from 6.8 to 36. Morphologically, the mudballs were mainly spherical (36 %) and disc-shaped (39 %), with an average particle size of 29.1-37.1 cm and sphericity index of 0.79. They also had an anomalous sorting mechanism, wherein, although the mudball particle size showed an increasing trend along the course, the sorting curve showed fluctuations due to the headward entrainment of mudflow and geomorphological disturbances. This study proposes, for the first time, a dual-threshold theoretical framework for mudflow development, which provides a high-precision criterion for the identification of erosion hotspots on the Loess Plateau. It has universal implications for the risk assessment of the global mudflow disaster chain and the design of prevention and control engineering.