Why Atlas Is Using Dual Engine Centaur For Starliner

Boeing's Starliner crew vehicle will be the first payload to use the Dual Engine Second Stage on the Atlas V since 2004. While the 2 engine version offers better payload to orbit, it's very expensive and the actual reason for the 2 engine requirement is a bit more subtle and stems from earlier plans for post-Shuttle crew vehicles.
NASA Spaceflight Forums has a lot more on this subject, including links to many of the documents I'm citing