It appears the motivation may be political games. From Politico Morning Ag Report:
NORTHEY NOMINATION IN LIMBO: Republican Sens. Ted Cruz and Mike Lee on Wednesday put a hold on the nomination of Bill Northey to a senior USDA post. Despite some issues with his official title last week (as Pro readers will recall), Northey’s nomination had been expected to breeze through the chamber. According to a spokesman for Lee, the senator had lifted his hold as of Wednesday afternoon, but a Republican Senate aide said Cruz's hold was still on as of late in the afternoon.
Payback in oil vs. corn war? Northey’s name had moved out of the Senate Agriculture Committee by voice vote, but multiple sources in the refining industry told Pro Energy’s Eric Wolff that the holdup was a "reaction" to the way Iowa Republican Sens. Joni Ernst and Chuck Grassley aggressively pressured the EPA into protecting Renewable Fuel Standard volume requirements, which helps biofuels like ethanol. "It's about time someone stood up to the kind of extortion we have seen coming from some of the Midwestern senators when the RFS is at issue," said one refining industry source.
Rubbing salt on wound: Blocking Northey has some extra sting. Grassley's grandson, Iowa State Rep. Pat Grassley, is a candidate to replace Northey as Iowa agriculture secretary if he is confirmed, the Des Moines Register reported. "It is indeed shocking that this type of corncob-rolling would be taking place in such an august body as the U.S. Senate," another refining source said.