Well here is a guess for you chaps....The Captain was flying and "between them" they managed to shut off the fuel to the wrong engine... and did not notice or if they did they were unable to recover the situation at their altitude in the time available.
How about
: here is a guess for you chaps....The Captain was flying and the fuel was contaminated.
How about
: here is a guess for you chaps....The First officer was flying and they had a birdstike in both engines.
How about
: here is a guess for you chaps....The Captain was flying and and the guages were in pounds not kilos.
How about
: here is a guess for you chaps....The First officer was flying and the fuel was mismanaged.
How about
: here is a guess for you chaps....The Captain was flying and maintenance had been carried out at some time prior to the flight that was identical to both engines.
How about
: here is a guess for you chaps....We wait for the outcome of the investigation.