They simply picked the release date announced by Tor's mother company McMillan in its fall 2010 trade catalogue for booksellers... That date has circulated for a short while now (the fall catalogue has just been released and it's also on the MacMillan website for online consulting).

October 26 is the official planned release date at the moment, even though Tor has not yet made a public announcement about it, beside telling the bookstore through the trade catalogue. That they give a date they are aiming for to bookstores but are waiting for a public press release might be a sign they're waiting first for more concrete information from Harriet and Brandon that everything is on track and they will make the deadline of Mid-August, or like for TGS it's the right time to tell the bookstores, but it's too early to publicize the book to the readers.

By the way, the release date for TGS was also "leaked" the same way last year. The temp cover art, the synopsis, the fact the book was to released in 3 books etc. All that information came first from the MacMillan trade catalogue, after some bookselling site had decided to put that "business" info online early. All that changed later was that Tor moved the date a week earlier than planned at the time the catalogue was published (a late change of mind, their first countdown gizmo also gave a november release date before it was changed to October 27).