As for the reasons that have led to this delay, the specifics are really unknown beyond word that the game simplye was not what makes a “great” game. Sony will be eating the cost of sending the title back to the drawning board, but for sure it is going to be right before the release it.
As for when Driveclub might be released, right now there is no word on that. We do know that the last release estimate was early 2014 and that has basically come and gone. We are hearing now that the possibility exists that it might not even make release this year and could be pushed into 2015. Sony promises to release specifics on an actual release date once they have something solid to announce.
Read Rohde’s interview with IGN here.