Looking around the interwebs, it appears there are a number of theories
A- He got his girlfriend (who goes to BYU) preggers
B- He wants to go on his Mormon mission
C- He expected to be starter from day 1 but has barely cracked the 3-deeps and he's pissed about it
D- He wanted to go somewhere else but his parents forced him to go to UCLA and he's pissed about it
E- He really does just have a personal issue going on and will be back in a couple days
He was there in spring and had a concussion the first day or two of spring ball that sidelined him the whole spring. Then he walks off after two days in fall. I think his commitment to football isn't there...so he's quitting.
Im sure his commitment will be there for BYU again in 3 years after a few years at a JC or door to door LDS'ing.
Edit (yes I had to edit that): hopefully it's not a girl friend issue, because Twitter stalking indicates he has one that would make irishdawg proud.