Why is expansion of rock important for magma to form?

The melting temperature of materials is affected by the pressure they are under. So when "rock" in the Earth's mantle experiences a decrease in confining pressure, not only does it expand, it's melting temperature drops. If the melting temperature of the material drops below the background (also known as the
in-situ
) temperature, then melting will occur and in this case magma will form.