What causes convection currents in the mantle?
Convection is relocation/circulation for stability, under conditions of different temperature and pressure, and density and viscosity..
Despite highs like Himalayas and Rocky Mountains, and lows like Mariana Trench in Pacific, the global surface is equipotential geoid and the gravity is normal to the mean surface level..
Yet, densities of minerals in the mantle differ, The same matter might have different viscous forms, at different depths, under conditions of temperatures, and pressure from above, that increase with depth.
So, once redistribution starts by convection, it is happening as relocation/circulation for stability, as a self-correcting process.