That is strange but at least you know it will be 100% when it goes back together. Be sure to use dielectric grease on the wiring harness connection on the fuel pump hanger and also inspect the fuel tank wiring connector to the body harness connector. It is located here:
Looks like this when the tank is out of the car (courtesy of Mustang5L5):
They are known to get water in them and have issues. Inspect, clean if necessary, put dielectric grease in there, and reassemble.