Sony has booked a massive 3-hour press conference for August 18th, the day before the conference kicks off for the general public. People are extremely curious about what Sony might reveal at this unlikely venue, especially since Microsoft and Nintendo have nowhere near the same presence on the GamesCom stage.
Traditionally, Europe has been a very stong market for Sony, so a mega announcement here does kind of make sense. Also, given the massive leaks that occurred just before E3, perhaps Sony wanted some time to prepare a proper announcement at a less frenetic event.