The solid, dividing strips serve as visual markers for the foreground. Once anything breaks out in font of them, blocking the view of the white dividers, the viewer's brain immediately translates this as a three-dimensional scene. The best 3D gif creators take note of this effect and tend to choose scenes from movies and clips from videos in which something is initially in the background and makes its way into the foreground, whether it be a person, animal, monster, or projectile object.
This begs the question, if I paint two lines on my glasses will I see the world in extra-3D?
Sources: viralnova.com, mymodernmet.com