check out this link for How a QB's target is chosen.
http://goallineblitz.com/game/forum_thread.pl?thread_id=2012862The QB calculates the "risk value" for each player based on several things (sort of in order-ish):
1) Being the primary receiver
2) How far down the checkdown list he is
3) How many defenders are nearby
4) How good those defenders are in comparison
5) How close the player is to the play's set distance (short/medium/long)
6) How close the player is to a first down (on 3rd/4th down plays)
7) The player's catching ability