Looks like my question has been answered:
The U.S. State Department is trying to persuade a senior Republican senator to lift a hold on the confirmation of Sung Kim, the nominee to become a new ambassador to South Korea, congressional sources said Monday.
Jon Kyl (R-AZ), assistant minority leader in the Senate, has been blocking the confirmation process for more than a month, according to the sources. He is known as a staunch conservative on foreign policy. [....]
“Sen. Kyl seems to be placing a hold on Kim’s confirmation due to the Obama administration’s North Korea policy, but he has not clarified the reason,” a source said, requesting anonymity. [Korea Times]
Apparently, the Senate custom is that hold letters are, at least initially, anonymous, and therefore unexplained.
I’m just glad to see that someone in the Senate is still looking over the State Department’s shoulder, now that Sam Brownback has gone back to Kansas.