In 2014 Batista won the Rumble. Reigns was the last person eliminated, and the crowd was cheering for him to win (everybody knew Batista was winning after he came back, and they didn't want it). The crowd was mostly behind Reigns as he kept getting a push the rest of the year. Then things took a bad turn in every direction. Bryan (the person they wanted all along) got hurt, so Cena took Bryan's place, dying at the hands of Brock at Summerslam. Survivor Series comes along and they wanted to do this big tag match to end the Authority at long last, and they wanted Reigns to get this big moment so you'd believe it when he won the belt, but Reigns got a hernia and couldn't do the match.
They reacted to this by giving Reigns satellite interviews every week where he'd talk about coming back, and he'd say the stupidest scripted jokes. The crowd hated it more and more. When he finally came back, he kept doing it while fighting the Big Show for months, with the crowd hating it more and more. And then he won the Rumble doing it, which the crowd expected, and hated, to the point they had Seth take Bryan's place and be the savior to try and calm the fans down. Except Seth kept looking like shit and got hurt, while Reigns still kept winning, doing long promos with jokes that the crowd couldn't care about, and main eventing. It was clear they were doing everything for Roman except the stuff that made fans like him in the first place, and the more he was seen, the more he did it, and the more upset the crowd became.
Finally it resulted in...this. His 3rd reign in 6 months, his 3rd big moment in half a year, being hyped as his Real Moment. Amidst a crowd of boos. Because he still talks too much, because he's still cocky in interviews and onscreen, and because they still won't change anything about him to make fans like him again because they want him so badly to be the new Cena, and Hogan, and Rock, and Austin all at once, forgetting that he's none of those. He's just Roman Reigns. For good and for bad, that's who he is. And if you just had him not say shit, this whole time, maybe none of this happens the way it does. As is, in a pisspoor Mania where everything went wrong, this was the only part fans expected completely, which is why it was booed most. It was the culmination of fans being told for 3 years they didn't care about what they wanted, to end an event where they were told it repeatedly for 7 unceasing hours.