United Flight 724 "encountered strong headwinds" and "out of an overabundance of caution, turned back about two hours in to the flight," a United spokesperson told ABC News, The rep said strong headwinds resulted in a "fuel overburn."
The flight, with 263 passengers and 12 crew members on board, landed safely at Honolulu airport Sunday.
ABC aviation expert John Nance explained that while an incident like this is rare, the pilots on board Flight 724 did exactly what they were supposed to do in this type of situation.
"When the headwinds are greater than what were expected, and are going to be sustained for four or five hours of flight, you're simply not going to be able to land with your legal minimum of fuel," said Nance.
"It doesn't mean you're going to run out, but it means you're not going to be legal. That's when you have to turn around," he added.
According to the Federal Aviation Administration, distance, the weight of the aircraft, the expected plane altitude, weather, and other factors are taken into account when making a minimum fuel calculation.
"Regulations for international operations require fuel on board at takeoff to include: fuel to the destination, plus fuel to the most distant alternate, plus enroute contingency based on a percentage of flight time, plus 30 minutes holding at 1500 feet above the airport," the FAA told ABC News.
According to Nance, while airlines have been very engaged for the past 20 years in trying to burn as little fuel as possible, that doesn't mean the airlines are gambling with passengers' lives.
"Airlines are not playing roulette by carrying as little fuel as they legal should. They are simply being efficient," Nance explained.
"But... [airlines] are using a lot of computer algorithms," that take into account weather patterns, "and if part of the [algorithm's] input is a certain headwind and [the headwind] gets much worse than you expected, then this can be the result," added Nance.
According to a United spokesperson, all passengers were booked overnight at a hotel and given meal vouchers. An additional flight was also added Monday morning to accommodate all passengers and get them to San Francisco, which departed at 11:42 a.m., according to a United spokesperson.
ABC News' Erin Dooley contributed to this report.