Over 1,000 workers at the Port of New York and New Jersey -- the largest on the East Coast -- went on strike Friday morning.

One problem with the strike: the Port Authority, which operates the port, says it isn't sure why workers decided not to show up.

"We don't know why they walked out," Steve Coleman, a spokesperson for the Port Authority, wrote in an email to CNNMoney.

In a statement, the Port Authority strongly urged port workers, affiliated with the International Longshoreman's Association, to return to work and that "differences" would be resolved afterward.

The Port Authority tweeted that no trucks should line up outside the port or be sent there for the time being: "We strongly urge the ILA members to return to work immediately and resolve their differences after they return."

The port workers' union did not immediately respond to comment. However, its spokesperson, Jim McNamara, told radio station 1010 WINS that port workers were unhappy about some hiring practices.

McNamara also criticized another organization that oversees the port, the Waterfront Commission of New York Harbor. The Waterfront Commission was created in 1953 to combat corruption at the port, according to its website. It did not immediately respond to a request for comment.

Related: U.S. economy comes to near halt at the end of 2015

Port strikes can be a big deal. Last year, the U.S. economy took a hit in the first quarter partially due to a port strike on the West Coast. U.S. Labor Secretary Tom Perez had to intervene to resolve the dispute.

U.S. growth in the first three months of last year, when strike took place, was an anemic 0.6%.

In 2014, the Port of New York and New Jersey handled $200 billion of goods last year and about 3.3 million containers, according to its website. By comparison, the port of Los Angeles, which had workers go on strike early last year, handled 8.3 million containers last year.