User blog comment:Saratje/PC release date explained - Skyrim: Dragonborn DLC/@comment-58.150.246.17-20130101005642/@comment-84.202.255.159-20130101173712

It wasn't it's own DLC, it was a part of the Dawnguard DLC that had to be separated due to deadlines. It wasn't really a small addition in itself, but the 2nd part of the Dawnguard DLC.