Haven't raised chicks in a while, but it looks like it might be a slipped tendon. If so you should be able to realign the tendon and wrap with vet wrap to keep it in place (but also making sure not to cut off circulation to the leg).
From the bitchin' chicken website:
"If caught in time (i.e. first 24 hrs) the remedy is to extend the leg backwards, hold the knee joint between your thumb & finger and roll it back and forth. If the tendon has slipped, you will feel it snap back into place (and possibly back out again, if the bone is not sufficiently developed). If you don’t feel the tendon pop in, your bird may instead have a rotated femur, which requires surgery. Once back in place you should use vetrap to hold it so the tendon stays in proper position. From my research sometimes this works and is a cure, and sometimes the tendon repeatedly slips and can’t be fixed."
I'd also make sure all the chicks are on a non-slip surface (helps prevent leg injuries) and maybe give a chick vitamin supplement as vitamin deficiencies can be a factor.