User blog comment:B214/High School DxD EX/@comment-26890593-20150821161049/@comment-74.197.121.121-20150827043630

From various spoilers and rough translations, the gist of EX that I got is that it bridges the alternate events in the BorN anime with the LN. Basically, because some evil being screwed up the past (I belive it's been speculated as Loki, if not some other being), it resulted in a gradual changing of the future (butterfly effect and all; apparently Asia is one of the casualties), so the kids are going back in time to fix the differences we see in the anime so it matches back up with the LNs.

If that's accurate, it's a pretty hilarious way for the author to indirectly fix the direction the anime took.