"Cage Crashers" is an orchestral piece from the Season 6 soundtrack.
Seamus falls down dead, and Sawyer notices that their cage's keys lie on his body. Kate stretches an arm through the bars to try to reach them, while Frank begins kicking the door, trying to break it open.
The survivors try to crash open the cage that imprisons them. The title likely references gate crashers, uninvited party guests.