According to the Washington Post , the cargo hold contained passengers' pets, although none of the animals were injured. The plane was flying from Los Cabos, Mexico, to Portland.

Alaska Airlines said in a statement "there was no indication to the crew that the door had been opened during the flight and all indications were that the door was partially open after landing."

"Our maintenance team inspected the aircraft, replaced the spring on the door, inspected the door and returned the aircraft to service," Alaska Airlines' statement concluded.

Alaska Airlines suffered a shocking incident in January when a window and part of the fuselage on one of its Boeing 737 MAX 9 planes exploded in mid-air shortly after takeoff from Portland, Oregon.

The explosion forced the plane to make an emergency landing. Fortunately, all 174 passengers and 6 crew members were safe.

While the airline appeared to downplay the issue, aviation expert Joe Schwieterman said the self-opening cargo door was a “pretty big flaw.” The plane that opened its cargo door was a Boeing 737 MAX 8.

"It affects a lot of the electrical equipment in that cargo hold. So it's a problem when you're flying an aircraft and something like this goes undetected," he said.

Both Boeing and Alaska Airlines are embroiled in a $1 billion lawsuit over the safety of their planes, and the chairman of the U.S. Transportation Safety Board has warned that the January incident could happen again.

Meanwhile, also last Friday, a Boeing 737 MAX 9 rolled off the runway at George Bush Intercontinental Airport in Houston, forcing all passengers to evacuate. The United Airlines flight was scheduled to fly to Memphis, Tenn.