User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-1.53.124.174-20150712141105/@comment-26520229-20150713051646

This pattern of events is hardly exclisive to the recent volumes like you claim. Antagonists in this series (and most series, for that matter), routinely appear out of the blue and interrupt the protagonist's everyday life for a brief, initial exchange or scuffle before the final battle that usually takes places elsewhere. Riser appears in the clubhouse in volume 2 discussing his intentions and humiliating Issei before planning a final battle (well in this case there is another battle afterwards). Freed appears in the town multiple times before his defeat in volume 6. Now those 2 examples are before the peace treaty and so there is no reason for enhanced security yet, but the plot formula is still the same for the most part. You have the same thing with Diodora and Loki in volumes 6 and 7. I grant you that neither of these two are considered antagonists before they appear and explain their intentions, but they nonetheless appear for a short introduction, disappear, and reappear for the climax at the end. The sacred gear users from the hero faction appear in the town in volume 7 and then are ultimately dealt with in volume 9 in Kyoto. The circumstances are always different, but there are only so many ways for enemies to enter the story; either they have to appear in front of the MC, or the MC appears in front of them (e.g. volume 16). Again, this has been happening all along in DxD and many other series too if you think about it.