For 53 weeks between November 2017 and November 2018, AJ Styles reigned as WWE champion. Along the way, he vanquished the likes of Jinder Mahal, Kevin Owens, Sami Zayn, Shinsuke Nakamura, and Samoa Joe, before getting downed by a heel-turned Daniel Bryan on an episode of SmackDown.
The title switch took place a week before the 2018 Survivor Series, and Bryan quickly slotted in to a PPV match with Universal champion Brock Lesnar, to accommodate the "similar champions face off" template of the recent Survivor Series'.
As for why Styles lost the belt to Bryan just before Survivor Series, the reigning IC titleholder gave his theory on a recent Twitch stream.
"I think the reason why you saw that happen was because you already saw Brock Lesnar/AJ Styles and - oh man, I'm just not going to go there," Styles said. "Not you guys, I'm sure you guys would like to have seen the rematch of AJ Styles and Brock Lesnar but [WWE] didn't think anyone else would."
Styles wrestled Lesnar at the 2017 Survivor Series under similar circumstances. Coincidentally, his winning of the belt came just before the pay-per-view, defeating Jinder Mahal, thus turning the Survivor Series bout from Lesnar/Mahal into Lesnar/Styles.
(Credit to Wrestling Inc for transcription)