Maybe this a bit of a long shot, but having been told of the sizable lumps of ice which have been known to form in aircraft fuel tanks over a number of flights, if the conditions are right (dissolved water freezing out, and large amounts of condensation from frequent descents through moisture laden air) could the imbalance have been caused by one of these melting? This might give an increase in fluid flow, although the fluid would be fuel with a significant water content from melted ice.
AIR790C: Considerations on Ice Formation in Aircraft Fuel Systems - SAE International
I'm struggling to see how ice could form or melt in one side only; however the way the aircraft was parked relative to the sun might explain that.
This is not the BA038 scenario, more a case of a build up from condensation over a number of flights in the outer part of the tanks from a succession of flights and then parking in conditions sufficiently cold that the ice build up did not melt until the flight in question.