Why do the A320's centre tank pumps stop working 5 minutes after the low level has been reached?
To elaborate on Ron's commment, the pumps are timed because the when the level of the center tank is depleted to the minimum level that can be sensed by a low level sensor, there is usually still residual fuel sloshing around that the pump is able to draw.  For both electric pumps and ejectors, you don't want them sucking air for extended periods, so you want to be able to fully deplete the tank without having the pumps run too long un-ported.

Airbus would have done testing to establish the average time to suck the tank completely dry, added a margin of some sort, then added the timer to the low level shutoff signal with the appropriate delay to achieve that end.