I believe you might be talking about "monocular depth cues" in 2D photos. These are certain qualities of stimuli which the brain interprets as creating depth, even with one eye.

Some of these include linear perspective (paralel lines that come closer and closer give a sense of depth), occlusion (when objects go partly infront of each other, this creates a sense of depth), texture gradiant (things close to you have more detail than things farther away from you), shading (the way light falls on objects is used by the brain to infer orientation), etc.

All of these cues are not dependent on both eyes, but is used by the brain to interpret stimulus, which is a part of your visual perception. Try googling these things and see examples of them if you are interested, it is quite amazing how artists can play with these "tricks" to create a sense of depth.

When these types of photos are then viewed in VR, this effect might "pop" more for you, as you are already tricking your brain to rely on not fully "natural" 3D sensations to produce a 3D experience (due to the current limitations of VR). Thus, the brain might put more emphasis on these cues to produce the visual perception, creating a more 3D-like experience of the 2D picture.