Thor's watching for signs of a feint, the obvious weakness of his blind side too strong for a determined foe to ignore - it's something he would take advantage of, were he in the Lion's shoes. Er, so to speak.

So when Aslan's trajectory suddenly changes, Thor sidesteps, just enough that the giant paw catches him on the shoulder instead, shoving him further into a half-turn. As he does, Thor clenches a fist and whips it sideways at where he last remembers seeing the Lion's nose, now fully out of his field of view.