Convection currents are defined as the transfer of heat due to the bulk movement of molecules within fluids.
From past experiences, we know that heat rises. When a liquid is heated from the bottom, it will rise to the top. To take the place of the previously heated liquid, the colder liquid that was at the top before will move to the bottom and will be heated. Then that will rise to the top and the cooler liquid that was being heated first will move beck to the bottom to be heated again. The earth's heat source is from the core, which is towards the bottom if youre looking at the mantle. So the same principle applies there as well.
