It began when Steam showed up among a list of icons underneath the new XIII portal.
So this has people convinced that at least Final Fantasy XIII is coming to Steam. This rumor gains plausibility from early statements from developers saying XIII was running on PCs (thereby making the 360 version possible), and the idea that running on a PC platform is the most economical way to make the Divein streaming service work...so why not release it commercially?
The other bit of detail was 'PC' briefly showing up as a platform for XIII on square-enix's online store, which was later removed.
Of course, XIII was not a good game and nobody wants the sequels, but this has people excited that another japanese developer could be riding the steam train as a way to tap into foreign markets where physical retail finds itself hobbled (europe in particular). Already folks are chomping at the bit for PC ports of all future SE HD games (XV, KH3, etc).
This is all speculative of course, nobody from SE is going to say anything officially until Fri/Sat when TGS gets rolling.