Earlier this year, United president Scott Kirby – who was formerly president at American Airlines – said his new carrier was “underperforming” in back-end financial deals, compared to his former employer and Delta Air Lines. Part of this was attributed to the launch of the Chase Sapphire Reserve, with a popular response from frequent travelers.
Note that the UA executive previously worked at AA. Further note that AA does not accept transfers from any USA-based consumer credit card program. (Starwood is a hotel program.) Recall also that UA only reluctantly agreed to participate in Ultimate Rewards, after the merger with CO. If one of UA's grievances is that Sapphire Reserve and other Ultimate Rewards cards are cannibalizing co-branded MileagePlus cards, we might see UA exit Ultimate Rewards, or some restriction on transfers.
History: When Ultimate Rewards launched in 2009 there were four transfer partners (BA, CO, IHG, Marriott). UA did not join until late 2011.