Oh, THAT'S what he said! When my daughter and I saw the movie the other day, I noticed that part. Gandalf pushed Aragorn and said something to him which caused a look of hurt, surprise and questioning on Aragorn's face, then Gandalf said "swords are no more use here". Now I understand the hurt look on Aragorn's face. Not just the being pushed by Gandalf, but the shove combined with the sharp admonition to "Do as I say!".
I think this section had some of the worst editing of the entire movie. In the book at this point Gandalf stays behind and attempts to block the balrog from passing through the door (not that he was sure it was a balrog yet). In the movie he tells Aragorn to lead the group ahead, but then he stays right with them all the way to the bridge.
BTW, one part I really missed in themovie was where Gandalf is confronting the balrog on the bridge and Aragorn, followed by Boromir, runs to support him. The image (in the book) of these pitifully weak men charging against such overwhelming power always affected me about as much as Sam's attack on Shelob. It also illustrates why Boromir was gradually accepting Aragorn as his leader.
(Pardon my getting out of sequence with regard to Shelob.)