Brian Plummer was sitting toward the back of a Delta Air Lines flight readying to depart La Guardia Airport on Monday when, he said, he noticed a man and a woman with a large service dog nearby.
The Atlanta-bound plane was not completely full, Mr. Plummer said, and the couple changed seats several times.
The man and woman never did settle in, and as Flight 462 began to taxi out to a runway, the man stood up, ignoring a flight attendant’s order to sit, saying that he had post-traumatic stress disorder, Mr. Plummer said.
A short time later, Mr. Plummer said, he felt the plane shudder to a stop. It was not immediately clear to him why, but the flight crew eventually told the passengers what had happened.
The man had forced open a cabin door, activating an emergency slide, and then he, his female companion and their dog slid their way out of the plane, officials said.
The couple was in custody; it was not clear what charges, if any, they might face, or whether they might be prohibited from flying with Delta in the future.
This was not the first time a panicky passenger has pulled such a maneuver at a New York-area airport, but neither Delta, the F.A.A. nor the Port Authority of New York and New Jersey, which operates La Guardia, could say on Monday just how unusual it was for someone to make such an escape.
“This doesn’t happen every day at the airport,” said Lenis Valens, an authority spokeswoman.
Mr. Plummer said he had noticed the man standing after the plane left the gate. “They seemed really nice,” Mr. Plummer said of the couple.
The man told a flight attendant that he could not sit down because of his post-traumatic stress disorder, Mr. Plummer said.
“If I sit down, I’ll freak out,” the man said, Mr. Plummer recalled.
The flight attendant again asked the man to take his seat, and again he refused, Mr. Plummer said. Instead, he began to walk toward the front of the plane, getting far enough that Mr. Plummer lost sight of him.
The plane, which had been scheduled to depart shortly before 10 a.m., returned to the gate, where the remaining passengers “deplaned normally and were accommodated on alternate flights,” a Delta spokeswoman said. (Mr. Plummer said his departure had been delayed several hours.)
No one was injured in the episode, the authorities said. It was not immediately clear where the service dog was.
The most famous example of an unauthorized sliding exit occurred 10 years ago at Kennedy International Airport, with a JetBlue flight attendant who said he had been involved in a dispute with a passenger.
In that instance, after announcing his intentions over the plane’s public address system, the flight attendant activated an emergency slide, slid down to the tarmac and threw his tie to the ground as he walked away.
More recently, a traveler aboard a Florida-bound flight at Newark Liberty International Airport fled through an emergency exit and down an inflatable slide while the plane was still at the gate in February 2018, according to WNBC.
When officers arrived to arrest him, the man was yelling that he did not belong on the plane because it was not his flight, the television station reported.