The decline in summer Arctic sea ice extent has been underway for several decades and is set to continue until summer Arctic sea ice disappears completely by the middle of the century, according to the latest climate projections. Based on observations and these climate projections, the rate of sea ice retreat is not linear: the decline in the Arctic sea ice cover is marked by periods of abrupt sea ice loss. Recent studies suggest that these rapid sea ice loss events (RILEs) will become increasingly likely in the coming decades. However, the processes driving RILEs remain poorly understood, in particular, the mechanisms controlling sea ice concentration evolution and the sources and sinks of sea ice mass during these climate events. Modeling tools that provide process-based analyses and budget decompositions across both historical and future climate simulations offer a robust framework for understanding the physical sources of these abrupt events. To investigate the processes behind these events, we characterize 31 September RILEs from the CESM2-lessmelt ensemble under the SSP3-7.0 forcing scenario across four periods spanning 1990–2065, and apply a two-stage process-based attribution framework to sea ice concentration and mass budget tendencies to identify the dominant mechanisms driving sea ice loss from a spatial perspective. RILEs are found to be a robust and recurrent feature of Arctic sea ice evolution, with a peak in occurrence around 2040, triggered across a wide range of initial sea ice states, from the thick and extensive cover of the early 1990s to the thin and reduced ice pack of the mid-21st century. Thermodynamic sea ice processes, and basal melt in particular, emerge as the dominant driver of the most intense and spatially significant sea ice losses during RILEs, while dynamic sea ice processes contribute a consistently minor fraction. Thermodynamic dominance strengthens as the century progresses and the ice pack thins, with dynamically dominated regions remaining confined to the vicinity of Greenland, the Canadian Archipelago, and the marginal ice zone. These results provide new process-level insight into the mechanisms of rapid summer Arctic sea ice loss and set the groundwork for the development of early warning indicators of the probability of RILE occurrence.