The 747 doors have two panels top and bottom which open first to equalise pressure should there be any residual pressurization. When the doors is closed, it's usually close from outside (if ground staff are available) so that these panels can be confirmed closed as it's not possible to check them from inside. If you get airborne with one open, it should close as the aircraft pressurizes. If it doesn't, I gather it gets a bit noisy. I suspect this is what happened in this case. A bit of a non event, but one which need to sorted by landing back.
This incidednt was also repoerted in this morning's South China Morning Post with tales of screaming passengers thinking they were going to die.
Incidently, there is a procedure for smoke removal in the 747 which has the doors opened partially in flight. Strops to hold them in position are carried in the aircraft.