How certain is a second explosion aboard HMS Hood?

Is it reasonable to doubt that HMS Hood exploded a second time as her aft magazines fell backward?

Yes. Secondary explosions were possible, but proving a distinct second major detonation would require much more evidence.

Propellant and ammunition could have produced secondary explosions as Hood broke apart. That possibility does not demonstrate a separate second major explosion after the initial catastrophe. The physical possibility of later detonations cannot by itself establish their scale, timing, or sequence. Resolving that distinction would require more evidence than the comment alone provides, making this a useful example of restraint when reconstructing a ship loss.