<p>Expansive soil landslides in reservoir-affected mountain slopes represent complex geohazards governed by coupled hydro-mechanical processes. However, the progressive deformation mechanisms under concurrent rainfall infiltration and reservoir water level (RWL) fluctuations—particularly the quantitative contribution of swelling pressure to failure progression—remain inadequately constrained. This study addresses this knowledge gap through an integrated investigation of the actively deforming Weijiapo landslide (Danjiangkou Reservoir, China), where synergistic rainfall-RWL interactions drive instability. Field monitoring data (GPS displacements, groundwater levels, precipitation, RWL) were synthesized with laboratory experiments and GeoStudio-based numerical modeling to develop a conceptual disaster cascade framework characterizing the rainfall-RWL-expansive soil coupling. A modified residual thrust stability model explicitly incorporating swelling pressure dynamics was formulated. Results demonstrate that rainfall primarily governs shallow fissure propagation and pore pressure response, while RWL fluctuations control deep-seated toe destabilization and saturation-induced shear failure. Critically, swelling pressure integration reduces the factor of safety by 18–32% under flood-level drawdown conditions. By establishing the hydro-mechanical linkage between external triggers and internal expansive responses, the expansion process was systematically delineated. A fitted time-history deformation function exhibits strong alignment with laboratory-observed shear strength attenuation (<i>R</i>² =0.92). These findings provide mechanistic insights for defining early-warning thresholds and predicting failure evolution in reservoir-affected mountainous terrain. This work substantiates the critical need to integrate expansive soil mechanics into geohazard assessment frameworks to enhance predictive accuracy and mitigation design in reservoir slopes.</p>