As high-end energy systems, including power generation & propulsion, continue to advance, fluid pressure and temperature levels tend to surpass the critical point and enter the supercritical fluid region. Under these conditions, fluids present significant localized alterations in thermophysical properties, which impact the flow dynamics. This can be, for example, strategically harnessed to induce turbulence in microducts, which instead are typically restricted to operate under laminar flow regimes. In turbulent square ducts, the localized anisotropic Reynolds stresses resulting from the fluid interaction with the perpendicular walls imply characteristic flow re-distributions that influence the core flow, known as Prandtl second-kind vortical structures. These streamwise vortical structures manifest near the corners of the duct and impose engineering challenges, such as in the prediction of pressure-drop and heat transfer. In this work, direct numerical simulations of a high-pressure transcritical square duct flow with distinct gravity directions in the cross-stream plane are performed. The buoyancy effects on mean fields are analyzed. Significant differences are highlighted in the distributions of secondary flow motions due to a strong coupling effect between the high-pressure transcritical phenomena and the buoyancy effects. The observed phenomena indicate that prior characterizations of secondary flows and their contribution to heat and mass transfer (commonly considered as independent from the Reynolds number) do not apply in cases above the critical regime where strong variations in thermophysical properties are found.