German night fighters carried rockets only during daylight interceptions, never at night. If the Halifax took part in a night raid that rules out the chance of a rocket, completely. And Ju88s never carried rockets in any form.
AA? Anti Aircraft, Air to Air? It's just a misuse of a term and creates confusion as there were no Anti Aircraft rockets used by German Anti Aircraft batteries.
Ammo cook-off itself is not particularly dangerous if the boxes do not happen to be in a confined space (such as fighter's wing) and the smaller the rounds, e.g. .303 the less dangerous they are. If a round cooks off there will not be "bullets whizzing about" since the casing tends to rip into pieces and the bullet as the single heaviest piece flies the less distance. But surely it would have been a spectacular fireworks display in the darkness.
All in all such damage could have caused even by a single 7.9 round in a right place but judging by the picture in Scherf's second link I'd say the weapon was most likely a 20mm in schräge musik configuration. The attacking aircraft could have been any German NF aircraft.
-C+
