Second and subsequent babies don't engage in the pelvis until right at the end, so it's not unusual for them to have an unstable lie right until then. Even breech babies can turn all of their own accord very late. My second dd was transverse (sideways) until the end and flipped while I was in labour. Your pelvis is a little roomier after you've already had a baby, and your uterus a little more stretched, so there's more room for somersaulting around