The plane, however, landed safely and there were no major injuries.
Still, the unexpected opening of the door caused some passengers to have breathing difficulties.
Several people were sent to a nearby hospital for treatment, a Daegu fire department official said.
Eight of those taken to hospital were schoolchildren from Jeju, according to Jeju's education office.
What we know about the incident
The Airbus A321-200 was carrying nearly 200 passengers as it approached the runway at Daegu International Airport, about 240 kilometres (149 miles) southeast of Seoul, on a domestic flight.
When the plane was still about 200 meters (650 feet) above the ground, a passenger sitting near the emergency exit "opened the door manually by touching the lever," the carrier's representative told AFP.
"The passenger has been taken to the police and is under questioning" to find out why they opened the door, Asiana said.
The Yonhap news agency shared a short video of the incident.
