I agree that was a very unlikely thing to happen but strong as they are, Aliens don't have super anchoring so they can be knocked back. I think she hit it twice, once to stun it and the second blow knocked it to the floor. I'll take another look now.
EDIT.
The Alien lunged for her and its own momentum knocked it into the wall when it is struck across the head. Emily then stunned it with a downward strike.