What causes light to be shifted?

It is the Doppler effect and is caused by the light source or the observer moving towards or away from each other. In basic terms, if the two are moving toward each other then the light waves arrive at the observer more closely packed than when they left the source. That is, the wavelength is shorter and the light becomes shifted towards the blue end of the spectrum. If the two source and observer are moving apart then you get a red shift.

This is analogous to the pitch of a siren which drops as the vehicle passes you and starts receding from you.