Former Uber CEO Travis Kalanick lately has gone on a hiring spree to staff his latest startup, CloudKitchens—snapping up at least four senior engineers from Uber. And that appears to have sparked an unusual warning from Uber Chairman Ron Sugar at the company’s latest board meeting two weeks ago.

Mr. Sugar told his fellow directors—who include Mr. Kalanick—that they should remember their “fiduciary duty” to the company, which includes not doing anything that would hurt Uber, said a person with direct knowledge of the board meeting. He didn’t mention anyone by name. But his comments came after he’d heard complaints from Uber management about Mr. Kalanick’s hiring of Uber employees, said two people with knowledge of the episode.