On January 14th, Kemp tweeted, "Received the copyedited manuscript of RIPTIDE yesterday. A few tweaks and the book is done." So this delay doesn't seem to be related to any issues with the writing of the book. As EUCantina speculates the push back was likely related to the fact that Timothy Zahn's new novel featuring Mara Jade and the Hand of Judgement will be released July 19th and would probably hurt the sales of Riptide.
EUCantina also speculates that the novel Shadow Games by Michael Reaves and Maya Bohnhoff will now be pushed back because this paperback is also now scheduled to be released on September 27th.
My speculation is that Shadow Games will have its release date pushed to October or early November. The only scheduled October release is The Complete Vader by Ryder Windham and Pete Vilmur. The Complete Vader will be a high end book, which if you can afford, you can also probably afford a paperback novel. So the placement of a paperback in the same month makes the most sense given the rest of the publishing calendar.
Sources: EUCantina.net and Random House
