It was a tense couple of moments just before noon local time Monday in the British territory of Gibraltar. All eyes were to the sky, staring in disbelief as a low-flying Airbus A320 violently tipped back and forth like a capsizing boat in high seas.

British Airways Flight 492 started like any other. It took off from London’s Heathrow Airport at 8:25 a.m., slated for a 2½-hour trip to Gibraltar International Airport.

Everything seemed normal until the final descent. The twin-engine aircraft was 2.2 miles west of the runway, about 375 feet above the Bay of Gibraltar, descending at 656 feet per minute. Without warning, the aircraft tossed left, then right, at times dipping 30 degrees or more.

“Suddenly we felt the plane drop and rock violently from side to side,” recalled Lisa Huxstep, a U.K. passenger onboard the flight. Huxstep was petrified as her fellow passengers and cabin crew went silent. But then it got worse. “My husband and I looked at each other in panic as other passengers started to scream. I gripped his hand and prayed.”

Huxstep said the plane endured the rocking nine times in either direction before leveling out. That’s when “the pilot thrusted the engines, and the rolling stopped,” Huxstep said. Flight data shows that the plane got as low as 300 feet — one football field — above the ground. Sixteen seconds after, the aircraft was clocked climbing at 369 feet per minute. The aircraft continued ascending as it passed the Rock of Gibraltar.

“We circled above the sea for five to eight minutes before it was announced we were being flown to Malaga instead, due to the strong winds,” Huxstep said. Flight 492 landed safely in Malaga, Spain, at 12:40 p.m. local time.

Since the airborne seesaw, few details have emerged as to possible causes. The empty aircraft returned to London at 7:29 p.m. with no mechanical issues reported. So could it have been the weather?

Despite the clear air, gusty winds out of the east at 25 to 30 mph prevailed most of the day. If blowing perpendicular to the runway, that could cause crosswinds that would make landing difficult. But in this case, they were blowing along the length of the runway.

What might have happened is a process called Von Kármán vortex shedding. When a tall object protrudes into a layer of relative uniform wind flow, alternating vortices spin up, downwind of the object. These whirlpools are often invisible but occasionally can be seen on satellite images in the wake of isolated islands.

It is plausible that this happened on a much smaller scale south of the airport. The Rock of Gibraltar stands 1,398 feet tall and is an isolated feature. With Monday’s wind speed and direction, it is possible that periodic eddies — perhaps 100 to 200 feet across, a wavelength perfect for tilting an airplane — formed continuously and propagated within a narrow strip downwind of the rock.

That would enhance or reduce lift over wings across short distances. The plane’s forward speed of 160 mph in the direction of vortex shedding made the alternating columns of upward and downward force arrive in rapid succession — and if the vortices were the right diameter, one side of the plane would be shoved upward while the other fell.

Another possibility? Mountain waves. The eastern slope — in this case the windward side — of the rock chain south of Gibraltar’s airport serves as a ramp that artificially forces air upward. By the time it reaches the top, it’s denser than the surrounding atmosphere. It sinks but then warms, rising again. These air pockets bubble up like a bobber in a pond. They are like the ripples that form behind a stone in a river.

Although no official explanation has been provided, one of these processes may well have been the triggering mechanism that spurred the plane’s oscillation.

British Airways offered little additional information, other than emphasizing that the “safety of [its] passengers” was its top priority.

“I don’t want compensation,” Huxstep said. “I want an apology. I hope some lessons were learned.”