Cockpit voice recorder captured one pilot asking, “Why did you cut off?” The other replied, “I did not do so”.
These switches have safety features to prevent accidental activation, and cockpit recordings show the pilots were surprised and confused by the change.
If a Boeing part were at fault, it would likely be the fuel control switches or their associated wiring/actuators.
If there were a mechanical or electrical fault, for example, a short circuit, faulty relay, or sticking actuator it could, in theory, cause the switches or the valves they control to move or close unintentionally, cutting off fuel to the engines