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Geochronology indicates a rapid transition from primary to secondary crust building on the Moon. However, a lack of consensus remains regarding the operative mechanisms responsible for generation of early secondary magmas and their global extent. This study tests the hypothesis that the earliest secondary crust (Mg-suite rocks) formed as a direct result of density-driven cumulate mantle overturn. To do this, we advance 3-D mantle convection models to predict the extent of lower mantle melting induced by cumulate overturn. In contrast to previous suggestions, our results demonstrate that mantle overturn alone can simultaneously explain the abundance, timing, and spatial extent of early secondary crust building without contributions from other energy sources, i.e., lunar KREEP (potassium, rare earth elements, phosphorus, radiogenic U, Th). We propose that gravitational instabilities in magma ocean cumulate piles are major driving forces for the onset of mantle convection and initial secondary crust building on differentiated bodies.