If you watch the replay, BYU sends 3 receivers to the left, Bohanon looks left to draw the defenders, then comes back to Roberts as he is crossing the field to the right.
No BYu receiver went into the flats or ran a short pattern to the right, so the DB backs up deep to help in coverage. If BYu had of releases a TE or ran a receiver quickly to the right, that DB would have stayed home and that pass would have been complete to Roberts.
Watched it 3 times and I am convinced the only reason the DB was there was because there were NO BYU receivers on that half of the field.