Nebraska, Boise, UCLA, and Missouri were not good his Freshman year.
Utah and Michigan were the only good defenses he played that year.
I admit, the Portland State game is a pretty big red flag, but it's 1 game, and the first game of the year. That said - I don't really know, I'm just hypothesizing that maybe he was never really very good to begin with (Which has been my opinion all along, which was kind of confirmed when the coaches started Taysom and continued to start Taysom through a lot of struggles all of last year). All I know is that I was not convinced by Mangum even after his Freshman year because it seemed, to me, that he mostly sucked against the quality defenses he faced.
And whenever BYU fans (Particularly gen-pop CBers) have a lot of fervor surrounding a new player, coach, etc, I tend to be immediately skeptical.