what happens at gally at the end of chapter 39 why does minho follow him

Thomas tells Gally to stop. Gally tells him to shut up, that he knows what Thomas did. A Griever has made its way partially into the room, one of its arms searching around. Thomas is frozen with fear until he realizes the Griever may grab Newt's unconscious body. However, Gally keeps talking, getting the Griever's attention. Gally tells Thomas not to go back to the real world. He tells him he doesn't want to remember. Gally gives Thomas a long look and then turns and hurls himself onto the Griever. The Griever grabs Gally and disappears.

Thomas runs to the window to watch the Gally move off with Gally's body, back into the Maze. He is sickened to the point of throwing up when he sees a lone figure pursuing the Grievers. It is Minho, running at full speed, disappearing into the Maze.