Humans likely to become Bound are ones who have already been in contact with the dead in one way or another. They may be sensitive to spiritual activity, can hear or see ghosts, can foretell another's death, or any other number of ghostly contact. Most potential Bound have no clue as they became sensitive to the words and deeds of the dead; for some, it seems to be an innate talent, while for others it might be a chance supernatural encounter or pure and simple luck.
Whether they treat their "gifts" as a blessing or curse, their ability somehow communicate with the dead is part of what allows them to return to life. The other part is a deep desire to remain living, a desperate call for help that attracts a Geist to their situation. The Geist snares the soul before it is beyond their reach and offers it a life if the potential Bound will provide its body for the Geist's use. Accepting the offer, often called the Event or the Bargain, allows the soul to return, now with the added burden of the Geist in tow.