under what circumstances will a single vertex split into multiple when UV unwrapping? I'm unwrapping a model, and a single vertex on the model is being represented as three (two joining onto one, are you can see in the screenshot) in the unwrapped view.

This is just one of four identical sections, and this is the only one that this happens to. So what could possibly cause this?
It can be caused by a seam on the edge marked with green:


You can mark and remove seams in edit mode by select edges and going to Mesh > Edges > Mark Seam or Clear Seam:



You can also call the functions from Edges menu - ctrl+e in the 3d viewport.