Andy Pasztor for the Wall Street Journal:
The shift, which surprised many aerospace experts and caught even senior Orbital officials flat footed, according to people familiar with the matter, underscores NASA’s desire to have greater assurance that the orbiting laboratory will have adequate supplies and material for experiments in the short term. A NASA announcement could come as quickly as the next few days, these people said.
My guess: no one jumped at the first of the RapidLaunch slots and NASA loves the idea of increased supplies with some lingering SpaceX unpredictability. Or ULA wants to use this sort of flight to prove the speed and efficiency of RapidLaunch. Or, with all the switching of Antares cores that has been going on at Wallops, maybe there is a problem they aren’t telling us about yet.