Hi KristianNorway,
It's a problem with the heating of the centre tank fuel by the air conditioning packs when on the ground.
"The criticism came as the board ruled that fuel vapors in the center wing tank of TWA Flight 800 had been heated to explosive levels because the air conditioning units on the 747 had run for more than two hours while the 747 was parked on the ground."
"The air conditioning units are located under the center fuel tank on Boeing-made planes, and the heat given off during ground operations can be get into the tank, raising the temperature of fuel vapors."
FAA to act on fuel tank hazard on Boeing 737s