I'm just speculating here, but maybe the Cubs decided that, with Lee serving his suspension, another position player is more important than a 12th pitcher for the next four games. Gallagher pitched 4 innings yesterday and therefore probably wouldn't be available for several days anyway, so maybe he was sent back down. It's also possible Fox is only up for today; Dempster is scheduled to be activated tomorrow, so Fox may go back down to make room for him.