Arizona Republican Jeff Flake halted an appeals court nominee last week and he’s not publicly saying why, a behind-the-scenes move that takes aim at one of Senate Majority Leader Mitch McConnell’s top priorities this year.

The Senate Judiciary Committee scuttled a planned vote June 14 on the nomination of Britt Grant, a pick from Georgia for the U.S. Court of Appeals for the 11th Circuit. Flake’s hold throws a wrench in what has been the Senate GOP’s smoothly operating judicial confirmation machine.

“Oh, it’s just something I’m working out,” Flake said Tuesday, without offering more details. The typically media-friendly senator did say his reason didn’t have anything to do with Grant herself.

The move has not gone over well with Sen. Johnny Isakson, who said he confronted Flake but doesn’t know the reason for the hold. “It’s an indiscriminate, irresponsible use of a privilege of the Senate,” the Georgia Republican said.

The issue could come up again as soon as Thursday, since Grant is listed again on the Judiciary Committee’s business agenda.