Interfacial preferential seepage poses a significant threat to the safety of check dams on the Chinese Loess Plateau; however, the quantitative relationship between this seepage processes and the initiation of dam failure remains unclear. Through combined field investigations and model testing, this study provides the first quantitative analysis of how the intensity of interfacial preferential seepage governs distinct failure modes and mechanisms in check dams. Key findings confirm the presence of a measurable preferential seepage along cut-fill interfaces, characterized by accelerated moisture migration and elevated pore water pressure relative to the surrounding soil. This imperceptible preferential seepage can trigger severe interface-concentrated failure, manifesting from particle-scale piping to structure-scale sliding. Crucially, we demonstrate that the compaction density of the dam controls the interfacial seepage intensity and thereby dictates the failure mode transition: from sudden piping driven by strong seepage under low density, to progressive sliding induced by moderate seepage at medium density, and finally to minor deformation caused by weak seepage under high density. Mechanistically, we propose a hydraulic-gravitational competition framework to explain these transitions: increasing compaction density shifts the dominant force from seepage-driven erosion to gravity-dominated shear, consequently altering the failure mode from piping to sliding. Furthermore, we established a technical pathway for translating model findings into field applications by developing scalable failure criteria for check dams. These findings offer the first mechanistic explanation of dam failure driven by interfacial preferential seepage, providing crucial insights for risk assessment of existing dams and the seepage-resistant design of new dams on the Chinese Loess Plateau.