Please don't discount that you have a much larger issue with your flock. I am a trained professional. I am also the first person to argue an internet diagnosis from FB or a forum, and I almost always do, so I'm not saying any of this lightly.
The leg split is a classic sign of Marek's and only Marek's, any Vet will tell you that. There's also a very weird symptom with a very virulent strain of Marek's that causes intermittent full body paralysis, usually only in broiler chickens.
Your state lab offers free consultations, call them in the morning and they will tell you the same thing I am: 207.581.3874
https://extension.umaine.edu/veterinarylab/poultry/
You can also listen to this podcast episode for more information:
https://www.breedersacademy.com/ep88-mareks-disease-with-dr-rodrigo-gallardo/
There are many possibilities for how they got infected, and you may never figure it out. The onset of laying can be stressful enough to cause a flare up. Vaccinated day-old chicks that come into contact with other chickens within 7 days can infect others. Something you can do is check the rest of them for tumors you might be able to see externally, weird patches of feather follicles, or something weird about anyone's eye. Any one of those things could mean that they are an asymptomatic carrier, but some never show any signs or symptoms at all. There is no cure for Marek's, and it can survive in the soil for years after the hosts are gone.
Good Luck.