I am not sure what would cause it, but I had a chick with splaid leg and took it to the vet and this is what I learned.
When the chicken has real splaid leg (I may be misspelling it) the bone in the join is rotating the wrong way as it grows, and as the chick gets older it worsens as it turns more and more. The vet said there are some surgical procedures but most don't work well as birds are so delicate and it would not be worth it with a barn yard chick.
It can happen in one joint (leg) or both and will continue to rotate until almost at a 90 degree angle.
Feel the joint you should be able to feel it working differently from a healthy chick (its good to have a comparison.) see if the joint is rotating wrong. if that is the issue you can do a few things
you can try to brace its legs but if this is the problem there is little point. there is a method with a band aid where you basically shackle the legs to each other so they can only spread apart enough to walk. or try to brace in some other way.
see if it learns to adapt with no intervention
or Cull. If both legs are splaid this is what I would do. If you look at my past threads I have nursed many birds back to health, I am not above tube feeding, slings and attempting home surgery. But if it is splaid leg there are really no good options because as the joint rotates the chick will no longer be able to walk at all, and with both, well it would be next to impossible for it to have any quality of life.
So sorry. maybe it is not splaid leg but you should be able to tell if it is. this can be a vitamin deficiency problem in the parents so if you have barnyard chickens you may want to make sure that the hens are getting everything they need to be healthy.
ETA Spraddle leg is different if it has that then the baind aid or braces could fix the problem.