Fuel pump wouldn't start until a quick crank after rebuild-- why?

[GN SBS]

Recently when putting the motor back together, when I first turned the ignition to on I did not hear the fuel pump energize as usual. A friend suggested that this is often the case until you give the motor a quick crank for the first time. Sure enough, it worked.

Just wondering why the brief crank caused the fuel pump to begin acting as it usually does when turning ignition to on.

Scott