Are fuel tanks separable from the aircraft or is it integral to the structure of the aircraft. Fuel is carried in the wings as a tank is it not?
External fuel tanks can be attached to wing hardpoints or to a centerline attachment point. These tanks are not an integral part of the aircraft, but modular units.
Occasionally, they may not be installed correctly (it's not an easy operation sometimes) and a pilot might be forced to eject them for reasons of safety, or they sometimes shake loose when not installed properly.
The same thing occasionally happens to explosive ordinance, as well. I've seen it so many times I've lost count.