In almost every Mac app I am aware of Cmd+UpArrow is a shortcut for "Move to top of file". AppCode uses it to jump into some kind of file chooser.
I think this is a really bad default behaviour. It goes against the grain and means that I have to learn an entirely new and unfamiliar behaviour for this app. I will remap this key but I think you just shouldn't do this kind of thing.
Edit: Or, rather, I would remap it if I could figure out what the mapping was, I can't see it in the KeyMap reference. It might be handy if the keymap editor allowed you to type a key and see what command it was mapped to. I have no idea what this is called.