One of my Wyandottes had this happen this weekend. I was trying to dust them because I think a couple have mites. When my husband caught her and turned her around so that I could do her vent area, this is what I saw. I cut off feathers that had dried on poo and then touched what was coming out of her vent gently. It went right back in without any hesitation and without any real force. We then put some olive oil in and around her vent area. I didn't really know what else to do, but after her dusting, she went on her mary old way. I gave them all some yogurt yesterday. I will check her when I get home. I hope that someone that has more experience gives you some insight into this! I sure would like to know what it is and what causes it as well. I hope your hen has a full recovery!