[cynwoody]

For all four engines to fail it had to be a fuel problem. Either bad fuel (water in the fuel) or empty fuel tanks. I'll bet on bad fuel.

According to the article, "TASS news agency cited sources saying a failure of the plane's fuel valves led to the failure of all four engines."

[MasterGunner01]

If the fuel transfer valves are electric and there was an electrical problem, that would point to why the four engines suffered from fuel starvation. My guess is these fuel transfer vales are controlled by the flight engineer.