Look again. The bird's head does not move backward at all, only forward. When you apparently see the head move backward it's actually keeping still while the body moves forward. The thinking is that the stationary head on a moving body stabilises the bird's vision. If it didn't happen, the world might seem blurred to the bird. There are some mysteries still, for example why it happens only in some birds and not others. Ducks and geese don't do it, which could be something to do with their short legs and waddling gait. Plenty of unanswered questions!