Electromagnetic waves consist of varying electric and magnetic fields.
Mountain tops are also above some weather and air pollution.
There is also less atmospheric absorption by water in the NIR window. There is usually less light pollution on top of a mountain, so the sky is darker and the stars are relatively brighter. Mountains around a valley block radio noise produced by humans, which could interfere with the weak radio signals from celestial objects. Since radio wavelengths are so large compared to visible wavelengths, a single radio telescope would have to be huge in order to have a resolving power smaller than a fraction of a degree. However, it is possible to link radio telescopes in different locations, and it would produce the same resolving power as having a dish with a diameter equal to the separation between the radio telescopes.