User blog comment:B214/Volume 23 discussion blog/@comment-6341273-20170324115421/@comment-29963364-20170324152951

@Pawn

I think that has more to do with Issei not actually being the one to activate his [Juggernaut Drive], but the past possessors reacting to his negative emotions and forcing the activation.

Issei had given to rage & despair from Asia's "death", and his mind went blank. The Boosted Gear simply reacted to those negative emotions, awakening the past possessors, who then recited the chant to activate JD.

So Issei's JD in vol.6 was just an "involuntary reaction." Therefore it was "incomplete." The catylist for activating the JD is "intense negative emotions", but the BG itself can only go so far. The possessor has to have the presence of mind to actually go the rest of the way, making the JD complete.

At least that's my speculation on it.