Daniel Bryan dunnit.
Per this week's Wrestling Observer Newsletter, Dave Meltzer writes:
"Another new match [for SummerSlam] is Roman Reigns vs. Daniel Bryan. The angle on Smackdown where all kinds of things collapsed on Reigns that looked so hokey is to lead to the revelation that Bryan was behind it. That also explains Bryan never saying anything about his career changing announcement."
The process of elimination essentially arrived at this, since Reigns and Daniel Bryan were the two biggest, most conspicuous absences from the major SummerSlam pay-per-view. And, while it's desperately disappointing that Bryan is now associated with the most abysmal angle of the year, Bryan is more than capable of removing the stench with a superb explanatory promo on next week's SmackDown.
Moreover, the match will be tremendous: Bryan is somehow even better than he was in 2015, when they met in a Fastlane banger, and the audience is far more receptive to the idea of cheering Reigns four years later.
Since Roman Reigns is marketed as the 'Big Dog', there's a scope for a sh*tty animal cruelty joke here, but than can be left to Corey Graves.