Fuel tanks where full at the same time as the GPU went off line? Causing the shutoff valves to open?
The Maintenance Manual says that the fuelling valves will close automatically if power is lost.
Could it be a sticky fuelling valve/s?
Depowered, the float switches would not stop the fuelling. The last line of defence would be the mechanical float valves on the venting system. The reliability of these are not 100%.
Did fuel come out of both wingtips?