So, here is my theory ---- The Losties get into a battle with the Freighter people.... a choice has to be made who gets on the helicopter to get away, and who stays. Kate ends up hurt somehow--- and Jack saves her and takes her on the helicopter. Before they get on, Sawyer sacrifices to stay behind (what does he have to live for off the island anyways??) and makes her promise to check on Clementine and let the baby mama know about the money he put in the bank for her.

Jack is obviously furious at Kate for doing anything for Sawyer because he saw them shagging in the bear cage.