While out on his mission to retain the commitment.
I think (could be totally wrong) that a signed letter is good for a year so other schools have wised up a little and ask them to sign again while he is out.
I agree though if someone is a little iffy on BYU sign with someone else, don't resign letter halfway through mission (if you are a 4-5 star recruit school will probably still want you when you get home), and then make a second decision when you return home.