Xander is touching up her outfit - he has shoe ideas, something he saw on In Skirts he wants to riff on - when the snake appears.
He sees it first because she can't see anything around her while in starscape. He pushes her out of the way - she doesn't realize what's going on in time to abort the reflex action of stopping herself on the way down - can't switch targets fast enough to stop the snake, when she notices it, to stop it from going on to eat her after it's already gotten him.
Then -