On instinct, Jack shields Ianto with his body, what with his being more durable if there's an impact. Thankfully, there isn't, so he steps away again.
Even with the dangerous situations the TARDIS got into in his times there, he never knew it to crash quite like that. It should be dematerializing, not dropping from the sky, and that's very alarming. Of course, just looking and sounding like the TARDIS doesn't mean it isn't some kind of trick.