I've heard that the pressure difference usually won't allow a door to open at altitude, but this was apparently on landing approach. They guy who opened the door has been arrested.
South Korea detains passenger after Asiana plane door opened mid-air
South Korean police on Friday detained a man who opened a door of an Asiana Airlines plane minutes before it was due to land in the city of Daegu, causing panic among the passengers, officials said.
www.reuters.com
Passenger opens Asiana airplane's emergency exit in midair over Daegu
An emergency exit of an Asiana Airlines plane carrying 194 people was opened right before landing at Daegu International Airport on Friday, causing breathing difficulties in nine passengers, but the plane landed safely, officials said.
www.koreatimes.co.kr