What are some explanations for the relationship between social and spatial mapping? Background: A study published last summer in the journal Neuron found that when participants were tasked with finding a new home and job by interacting with characters in a virtual roleplaying game, activation in the hippocampus revealed tracking of social relationships through two dimensional vectors based on power and affiliation. In essence, they navigated through social space just as they would a physical environment: by using a cognitive map. Original article: 

Tavares, R. M., Mendelsohn, A., Grossman, Y., Williams, C. H., Shapiro, M., Trope, Y., &amp; Schiller, D. (2015). A map for social navigation in the human brain. Neuron 87: 231-243. Retrieved from http://dx.doi.org/10.1016/j.neuron.2015.06.011.

Question: What are some hypotheses as to why this relationship exists? 

Side note: If anyone is interested in participating, I am completing a research study to investigate the relationship between social and spatial skills. Here's the link for that: http://www.surveygizmo.com/s3/2620527/Social-and-Spatial-Skills-Assessment
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.