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.