What causes a car horn to change pitch as the car passes you?

This is caused by the doppler effect.  The sound from the car horn or anything else comes from a sound wave.  Waves repeat themselves and is measured in cycles per second.  Picture an ocean wave moving up and down and repeating.  Each complete up and down motion is a cycle. As the car approaches you, the frequency (or cycles per second) increases, and as it passes you, it decreases.  This change in frequency has a quality that we detect as the change in pitch.

THE doppler affect CHANGES THE  frequency of the sound waves as it passes you.