When you move your head from side to side or walk around, the apparent movement of stationary objects relative to one another is .

Motion parallax cue.

Explanation:

Motion parallax cue can be defined as a monocular depth cue that arises from objects moving at constant velocity (speed) across the retina or frame appearing to move faster when they are closer to the observer than they appear if they were at a farther distance.

Simply stated, motion parallax refers to a change in position due to the movement of the observer.

Hence, when you move your head from side to side or walk around, the apparent movement of stationary objects relative to one another is known as motion parallax cue.

Therefore, monocular depth cues such as motion parallax gives a valid explanation as to why stationary objects such as trees appear to move when you move your head from side to side.

For example, an observer (passenger) in a moving car looking outside the window would see stationary objects moving due to motion parallax cue.