User blog comment:B214/Question and Answers 6/@comment-118.172.205.177-20160711144501/@comment-26345952-20160712043507

Maybe the God of the Bible didn't inteded to happen but He also never did anything about it either. Maybe he saw potential on this bug in the system or knew what this bug will produce for humans. Regadless of weather he did it intentionaly or not is still a mystery.

However the JD is also a sort of a mystery. It is sealed but realesed when negative emotions hit max and then let the user enter willingly, similar to the BxB. I say both because there were only two people that had made Vali enter Juggernaut Drive by anger, but he was willing to eneter the JD on Vol. 4 at the end of the fight with Issei (mostly because of interest or adrenaline) even if Albion was against it; if it wasn't for Bikou, who abruptly showed up, Vali would have entered the JD.

I think that it has to play with the emotion. Perhaps Issei's was incomplete because he did not surrendered himself completly to Ddraig's power wich showed his sadness, or perhaps not. Perhaps it was that he eneter it in a subcuncious level and with broken mind state (due to the supposed death of Asia). I dont know your guess is as good of a guess as mine.