It could even have been a drop tank. Hung Ordnance refers to something that (a) is designed to be detachable (dropped or fired) and (b) failed to release when commanded or (c) appears to have partially released even if not commanded. It does not have to be "live" ordnance, although the danger is less.
An "empty" drop tank (they don't drop them over populated areas, just fly them home empty) is more dangerous than a full one, due to the vapors. If it's hanging by one end, there's a danger of it breaking loose and skidding down the runway on landing, or dragging the free end - either of which can create the big fireball.
Likewise, even an inert (empty) missile body or solid dummy bomb is still a projectile hazard if it comes completely free (likeliest at the "bounce" of landing) and then can bounce into the aircraft itself, spin off and hit another aircraft, or just come apart and become a "foreign object" hazard, where pieces become a danger to the next aircraft to land (blown tires, injested into engines, etc.)
Add in that "live" ordnance has even greater explosive properties (HE in bombs and missiles, fuel even in "target" shot missiles) and you can see where the airport safety people and the fire department can get pretty nervous pretty quick.