I actually have a source that directly contradicts this.

Before the season he was leaning a hard BYU and still hasn't ruled them out but this year has made them less excited.  Both Tanner and Dad want BYU but also want what is best for him and current issues with Sitake (they are good with Detmer and offense) are more likely to push them elsewhere.

Same issues as others have discussed: accountability, excitement on sidelines, environment, etc.

Down year doesn't bother them as much as what they see from the coaching staff starting at the top.

If I were him I wouldn't sign with anyone, go on his mission, see what the landscape looks like when he gets home and sign then.