It's a case of so called standing waves, e.g. like those that appear in a flute when blown. In this case, it's not the air that flows in a tube, but electromagnetic waves traveling around the Earth between its surface and the upper atmosphere (ionosphere), and as the Earth has about 40000 km around at the equator, and the electromagnetic waves travel at about the light speed, i.e. 300000 km/sec, they make about 300000/40000 = 7.5 (7.83 more exactly) circles around the Earth in one second, hence the frequency.
Not unlike in a flute, where closing different holes leads to different sounds, changes in local conditions around the Earth due to lighting etc. lead to changes of this frequency.
I don't think we have anything to fear from these changes - they don't happen so swiftly, and we will probably be able to adapt to them.
Sleep tight!
