Please Help! Chicken almost dead then comes back to life with TLC.. I don't understand what is happe

Can someone please help me with this situation I've been put in? I just can't fathom it..

About a month ago I found one of my chickens outside in the pasture "dead". I picked her up and her neck was limp as if she was dead but she was surely alive. I took her in the warm house knowing she's going to die. In the time that I had with her I encouraged her to drink by putting her beak in the water. She couldn't hold her head up at all and couldn't stand- she would just lay there but she tried so hard to lift her head. She also acted like she was in pain. She would puff up her feathers suddenly every now and then; she almost acted like she was trying to be a peacock.. A few days passed and she started sitting on her own and trying to hold her head up. Next thing you know she is drinking on her own and even eating on her own with her head up! She would try to walk but couldn't go very far. Today, she is holding up her own head, eating, drinking, walking and clucking like a good healthy chicken.

I guess what I am asking is there something that caused it? It hadn't gotten very cold outside at the time I can't think of what could've been the problem. No fighting, no animals got in their pen, she hadn't been stepped on, she had plenty of feed and water..

She Almost died
Couldn't hold her head up
She was limp
Wouldn't eat drink or walk
Now she's doing just fine like nothing happened.

I'm so curious to know, but I also went out this morning to find another chicken with the same symptoms.. Hopefully I can get her to pull through. Any thoughts are great.. and thank you in advance!