It seems like Kate is clearly the most traumatized by Jack's death, so I got thinking... what if Jack was on the flight (business? maybe to fly to Rebecca on tour? - could be any number of reasons) and before it crashed, he was somehow able to call the house? Maybe Kate was on the receiving end and she was the one that got to speak to her father, minutes, maybe seconds, before his death. This would explain the trauma and close connection that she has to Jack's death, and it would be one hell of a tear-jerker."