Because she's using the armored core grip, an old Japanese meme spawned from some of the games having a default control scheme that made it impossible, or at least very difficult, to actually use all of the game's features holding the controller normally. It's usually claimed that you needed more than two thumbs to actually play the game, hence this silly joke about holding the controller backwards.
Having said all that, I'm not sure if that actually warrants tagging armored core here.
