The following is all a firsthand account from CM Punk, speaking in 2014 on Colt Cabana's the Art of Wrestling podcast.
It is not necessarily what happened, OK Ryback?
CM Punk was allegedly once injured by Ryback's alleged inability to protect his opponent. Ryback denies this. Punk allegedly chewed Ryback out backstage, allegedly asking whether Ryback had done it deliberately, or if he was just as "dumb as f*ck".
With no alleged explanation nor comeback to crack, Ryback allegedly said "I'm dumb as f*ck".
Whether accurately or otherwise, this version of events became the accepted truth: given what Cody and Daniel Bryan have said of their time running the roads with the Big Guy, it's an entirely plausible response. And Cody and Bryan may have embellished a little, Ryback, sorry, but they reckon the Big Guy thought Terminator 2 was so-called because there were two terminators in it. Ryback was unaware of the concept of movie sequels, and they presumably intimidate him now.
With his old cyborg wrestling character, he's into robots and sci-fi, but where on earth is he going to find the time to watch the other 2048 Blade Runners?