Although double loading sounds like the reason, TV station interview of the shooter and cameraman still didn't sound exactly clear as to the cause. They seem ignorant (if that's the correct word) as stating something/obstruction in the bore. Being the rifle has an added muzzlebreak and the cameraman works for a rifle company - makes me pessimistic of the real truth.
Duplexing for ML with Black is to ignite the smokeless powder - not recommended by anyone.
Duplexing for BP cartridge is completely different - smokeless over the primer then Black; to keep the throat of the chamber clean for easy loading of the next cartridge. This has been outlawed in completion by the NRA BP for quite a few years (5 or so) due to guns blowing up, now it is Black only (or Pyrodex is the only sub allowed - which is quite an old rule).