You listed scratch grains and BOSS as part of your chicken's regular diet. You should make sure that they are primarily eating the layer pellets. Scratch grains and BOSS are treats.
I have never had a problem with yogurt causing diarrhea. If you gave them a LOT of it, I suppose it could have caused a problem. I always stir the yogurt into a little layer crumbles and make it about the consistency of thick oatmeal whenever I serve it to my girls. I don't give it often though.
I would suggest removing all treats for a day or two and see if the poo goes back to normal. Then reintroduce treats one at a time, and just a little to see what happens.