According to Chase it's random, but who really knows.
Well, if that were true, Chase had better hire new people for their marketing department.
What marketing strategy is there in soliciting massive bonuses to random people?
Amex seems to have shed some light on how they target customers for bonuses - "first time cardholders only". (Offer a handsome bonus to lure new people into the Amex network.)
Anyhow, on forums like FT, people are suggesting to "opt in" for credit card offers at the FTC website.