From what I can tell it seems to be based on inbound links (and search query specific, to a point, I think? lol) ... I thought inbound links and on-page text, but the more I'm reading and making on-page changes with no effect on the title in the SERPs the more I think the link lovers are at it again and using inbound link text (on and off site) to make title changes.
Just my thoughts ... I don't have any hard data to back it up, except a lack of change to the SERPs title from on-page changes ... I should know if the most recent round had any effect in a day or two.
I'm also starting to wonder if the on-page title change and subsequent SERP title change to a second 'GoogleTitle' noted by flashdash above is just a coincidence and would have happened without the on-page title change?