The teaser videos were made when the game wasn't completed yet, so things like that will probably change. The AT-AT probably wasn't moving becuase the person playing the game told the AT-AT to go to that spot and stop or something, becuase I know AT-ATs can move while shooting. As for the imp. troops, they were running away and once the rancer caught them, they decided to kill it becuase they new they couldn't run away anymore.