The funny thing about the tomahawk chop is that last year during the National League Division Series (Cubs vs. Braves) many Cub fans made the trip to Atlanta because tickets were easy to get. I guess the Braves' fans are so spoiled the Division Series doesn't mean much to them.
Anyhow, once the Cubs started spanking the Baraves like the red-headed step-children they are, the Cub fan took off their hats and hit the tomahawk chop thing thus mocking the Atlanta fans.