Several people were injured on the -200 series in the early days by the situation you describe.
The hydraulics started up and the nose wheel turned the tow-bar after the tractor had disconnected injuring the engineer/ground handler doing the push-back.
Shortly thereafter Boeing introduced the nosewheel steering lockout pin which isolated the nosewheel steering until the tow bar was removed.
Various changes have been made over the years and I don't know if this is still the case on the NG generation, but that was the origin of the story.