Educated guess is "hand error". There has been the occasional 747 incident/accident where the PF inadvertently only grabs the reverse piggyback levers on 3 engines - missing the furthest thrust lever (so left seat misses #4, right seat misses #1). As they command reverse, they accidentally nudge the other thrust lever forward and the resultant asymmetry thrust pushes them off the runway.
I'm aware of at least two previous events - coincidentally both in Korea (although only one was Korean Air). Pretty exciting ride, but no one seriously hurt.
If, indeed, it was "Uncommanded High Thrust" (UHT) due to a failure in the engine control system, it would be a very rare event (only one known UHT event on a 747-400) and the Feds will be all over it.
In fact, if it does turn out to be UHT, I may get a phone call
