Actually Vortex, the scavenge pump first run was triggered by the CWT Over ride/jett pump LP warning at quite low fuel levels - 600 - 700Kg if I remember correctly. It is triggered again by the transfer of fuel from the R tanks. Electric scavenge pumps move fuel to #2M, but later aircraft have jet pumps which scavenge to both #2M and #3M and actually empty the CWT.
The stab fuel is left there for T/O and climb to ensure that the CG remains always predictable
AFAIK.
Just BTW, the pumps are selected to make the fuel feeds so, it is not automatic in the main but the system does give us EICAS messages.