Isn't that awesome?
It's caused by particles moving faster than the speed of light in a particular dielectric medium. Usually you see it in nuclear reactors caused by beta particles in water.
It's easy to get tricked by the wording. Nothing that we know of moves faster than c (the speed of light in a vacuum). That doesn't imply that light is the fastest thing in any given medium, however.
The concept is similar to a sonic boom. A bunch of wave fronts build up and (thanks to the Doppler effect) eventually get shifted into the visual range of light.
I love that shade of blue 8)