I didn't say they ran out of fuel, but there are only two things that can practically fail both engines at the same time. One is fuel starvation caused by not having enough fuel or mismanaging what you have.... very difficult to do in a 777 and there appears to have been no mayday call or low fuel advisory to ATC.
The other thing is contaminated fuel. Considering the aircraft was out of Beijing, this would be the first thing I would be checking if I was in the management of any airline flying out of there.
If it wasn't one of those two things, then I am stumped.