http://spacenews.com/nasa-signs-agreement-with-boeing-for-soyuz-seats/
NASA signs agreement with Boeing for Soyuz seats
February 28, 2017

https://www.sncorp.com/press-releases/snc-dc-ula-statement/
SIERRA NEVADA CORPORATION SELECTS UNITED LAUNCH ALLIANCE
ATLAS V FOR FIRST TWO DREAM CHASERR SPACECRAFT MISSIONS
July 19, 2017

BoeingがStarlinerを中止するリスクを考慮したんじゃない?