Due to variations in the density of the atmosphere, electromagnetic radiation is refracted according to the varying refractive index with height. Variations decreases linearly with height under standard conditions (no localized disturbances). As a result, the wave gradually curves until, if the wavelength and angle are correct, the signal bends back toward the earth (see graphic below).
As with visible light waves, radio waves must equal a critical angle in order to be transmitted between regions of different refractive indices. When less than the critical angle is experienced, the waves are bent at the interface back into the medium from which they originate. In the case of an abrupt interface like air and water in a glass, the bending is equally abrupt. Layers of the atmosphere present a much more gradual interface so the reflection tends to exhibit a more curved shape.
This image shows the familiar pencil in a glass of water.