What are some explanations for the relationship between social and spatial mapping?
I understand you are asking why there would be a relationship between social space and physical, spatial environment, via a cognitive map. 

That would be owing to humans naturally mapping environments as well as experience (Tolman made his name on a natural phenomenon). Lakoff claimed that mapping for temporal orientation was a biologically determined process.

Radden (2013) agrees that humans operate a spatio-physical “image content” in order to “objectify” experience.

As I cannot agree that suffering or danger bring heightened awareness and high information processing, whereas routine activities would be “situations of low information processing” (Radden), dimensionality stirs my further opposition. :)

Time encourages Radden to terms as "zero-dimension". Your quote emphasizes two-dimensional Euclidean spaces.

"Zero dimension" would not be perceivable, therefore unfit for human natural mapping. Two-dimensional correlates would have resulted from the framework of a computer game talking bubbles (as the authors admit), rather than natural hippocampal function, evolved in three-dimensional space.

Obviously, brainstem and neocortical performance should not be omitted from a picture for real social interaction.