Donald Trump’s tirade on Twitter against aircraft manufacturer Boeing sent the company’s stock tumbling nearly $2 a share before markets opened on Tuesday, leaving many to wonder what prompted the president-elect’s sudden criticism.

Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion. Cancel order! — Donald J. Trump (@realDonaldTrump) December 6, 2016

Trump called for the U.S. Air Force to cancel Boeing’s contract related to the production of two new, highly customized Boeing 747-8 aircraft designed to fly future presidents of the United States.

Boeing’s current contract is for $170 million, which does not include the cost of “purchasing or modifying the airplanes as necessary to deliver Air Force One,” a spokesman for the company told the Daily Dot. The company declined to comment on the possible motives of Trump’s tweet, which claimed, “costs are out of control, more than $4 billion.”

The answer may lie in a Chicago Tribune column published less than 30 minutes before Trump’s tweet, one that reflected Boeing CEO Dennis Muilenburg’s apprehension over Trump’s indelicate handling of U.S.–China relations.

According to the Tribune, Muilenburg is urging Trump and the GOP to tone down the inflammatory rhetoric hurled at China over the past week. (During the diplomatic row over Trump’s controversial Taiwan phone call, an advisor to the president-elect told reporters that “if China doesn’t like it, screw ‘em.”)

Muilenburg argues—correctly—that international trade is essential to the U.S. economy; roughly 11.5 million American jobs are supported by U.S. exports, according to the International Trade Administration.

A third of the 495 Boeing 737 airliners manufactured at the company’s Renton, Washington, plant last year were bound for China, Muilenburg said. “And about a quarter of all our airplane deliveries … were bound for Chinese customers.” As the Tribune reports, U.S. jobs hang in the balance:

A steady influx of China commercial jet orders means more work for Boeing’s thousands of U.S. workers while also providing lift for its network of domestic parts supplier and vendors. Around 90 percent of Boeing’s total workforce of 151,000 people and 80 percent of its thousands of suppliers are U.S.-based, according to Boeing.

It’s unprecedented for a president-elect to nonchalantly target a major U.S. company during an apparent retaliatory tantrum—but such a fit is in no way unusual for Trump, the real estate mogul turned reality TV star turned U.S. leader.

In early October, candidate Trump promised, once elected, to use his influence and power to retaliate against conservative lawmakers who didn’t support him after a video surfaced in which Trump boasted about getting away with sexual assault.

It’s unclear where Trump got his “$4 billion” figure, though it could be on the mark. The Government Accountability Office said last year the cost of the project, including research, development, and production, could reach $3.2 billion. The Air Force would still have to purchase the two aircraft, ostensibly shooting the final cost over $4 billion.

Whether or not the hefty price tag is in fact too much for a pair of planes meant to protect the life of U.S. presidents for decades is a different story. “We look forward,” Boeing said in a statement, “to working with the U.S. Air Force on subsequent phases of the program to deliver the best planes for the president at the best value for the American taxpayer.”