I tried my best to imitate your ground destroyer...
lol, haha. Probably someone set the ammo on fire after they attempted to fire the main gun at the enemy, but since the main gun can't detach from the tank (Since the main gun is basically 70% of the tank), the ammo ignited and fired off inside the barrel, but that damaged the barrel and the radioactive warhead on it exploded.
ground destroyer backfire? or did someone decide to set it on fire once the commander yelled fire?
Just like the Obusier de 520 modele 1916