Let's just get this one out of the way early, because you knew it would be here.
Anyone who has watched WWE lately knows that they've been aggressively pushing Roman Reigns as the new face of the company for years now. He has main-evented the last four WrestleManias, he has made it to the final three in the last five Royal Rumbles (with one of those Rumbles being built around him entirely), he's been put over hot babyfaces like Daniel Bryan at Fastlane 2015 and Braun Strowman last year, and he even beat The Undertaker at WrestleMania 33.
What's worse is that WWE hasn't been consistent with the way they portray him. How can one be both a "Big Dog" and an underdog? For that matter, how can someone who rarely (if ever) loses be praised for "defying the odds"? How can we sympathize with a man who is always in the spotlight?
This is not to say that Roman Reigns is undeserving of his spot, per sé - it's to say that the fans are reacting adversely to Reigns because of how inorganic his push has been thus far.