As I heard it, its due to the 747 explosion that happened (mentioned above), with a wiring/fuel pump fault in the centre tank.
The problem comes I believe when you have little quantities of fuel in the centre tank, the air fuel mixture is apparently good for an explosion, if there were to be a re-occourance in the wiring fault/pump fault.
To counter this, were told when fueling aircraft, when the centre tank is to contain any fuel, it must be a minimum of 2 tons. (Above the critical AFR).
If we have an AC with a wing capacity of 6tons/wing. And we were going flying with 13 tons of fuel to our destination, we cannot have full wings and one ton in the centre.
We would pre-load the centre with 2 tons, and then put 5.5 tons in each of the wings.