Right...See if you can make sense of this.
The centre tank pumps have a higher output than the wing tank pumps, which is why the fuel in the centre tank is automatically used first. With one centre pump inop the pressure in the fuel manifold is still provided by the remaining higher rated centre pump. The wing pumps are "idleing" if you like and so not pumping any fuel into the feed manifold, they are waiting for the centre pump pressure to drop below their output rating. With one centre pump running and the crossfeed open the pump is feeding both engines from the centre tank (with the crossfeed closed the pump will only feed its associated engine and the other engine will be fed by its associated wing tank pump). Once the centre fuel has been used its pump can be switched off and the crossfeed closed. So if the crossfeed is closed with one centre tank pump switched on, you"ll soon have a fuel imbalance.