I think.....it may.....be one of 2 things
Could just be that the parents had terrible conformation....
Or I think it could be the result of an acute joint ill....which sometimes affects the back legs rather than the front. I had a lamb like that about 14 years ago and it never actually went off its feet, just walked stiffly and looked a bit wasted behind.....but caught up eventually.
It may not have had the best start not enough or any colostrum and may always be a sickly lamb. But good luck to her for having a go, it looks bright enough!