Seth had been out the night before and missed the flight. After taking a nap in the passenger lounge, he awoke to a TV report about the first plane hitting the tower. After the second plane hit, the airline announced which flights had been involved and he realised he was supposed to be on the plane that crashed into the North Tower.
Mark on the other hand, was invited to attend a film festival in Toronto and decided at the last minute to charter a jet with friends. He later caused controversy by saying that he would have prevented the terrorist attack from happening had he been on the flight.