Does 屁股 refer to the womens' bottoms in 几个包着头巾的女人正将棉秆拔出来，她们不时抖动着屁股摔去根须上的泥巴?
摔去根须上的泥巴 means to swing cotton stalks 棉秆 and hit ground heavily in order to remove the mud attached on the root. In order to make sure the root can hit the ground, they have to squat or bend their knees. By repeating the motion several times, the mud can be removed thoroughly. So, looking behind, their butts shake up and down.