If the ball had landed outside the boundary without bouncing first, the batsman would have scored the maximum possible runs from a single shot - 6. If the fielder catches the ball, he's out. However, if the fielder catches the ball but then steps outside the boundary, the batsman scores 6 runs again.
What you see here is the fielder attempt to catch the ball, but realise that he can't do that without stepping the boundary. His teammate has read the situation and is supporting him within the boundary. When the initial catcher pushes the ball to his teammate (while still in mid-air) the catch is made and the batsman is out.