I have several birds in my flock who 'moonwalk' (I like the expression - I'd never thought of it like that before!) They put their heads down and walk backwards for several steps, particularly when in a stressful situation.
I wasn't certain what it was, but last summer I lost two young birds quite suddenly with severe neurological symptoms, and after necropsies the vet told me it was most probably Marek's disease. Only two birds from ten or so hatchings last year developed the symptoms (both had to be euthanised due to the sudden onset and severity of the symptoms - unable to walk, then unable to stand, then unable to hold themselves upright even when sitting down, and all this developing within 12 - 24 hours).
I now believe that the birds I have that 'moonwalk' are those that have been exposed to the Marek's virus but have been able to resist it to a certain extent. They tend to have a lot of head twitching (especially on the perch in the night) but otherwise live full and happy lives. They are productive layers and fully integrated members of the flock (one is even the top chook), so as long as they are living good lives I am happy to let them be. In fact I have hatched some of their eggs, and the chicks have never displayed any neurological symptoms - maybe a case of natural resistance to the disease? (I know that there are threads on BYC about breeding for resistance to Marek's, but I don't keep detailed enough records to have anything useful to contribute to them.)
I would therefore be very inclined to believe that Marek's could be the cause of your bird's symptoms. Some birds are not affected at all, some can be affected and recover, others suffer mild symptoms for a long time, and some deteriorate rapidly. Nambroth's article
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq is a very informative page.
There is very little you can do to avoid Marek's - it is carried by wild birds and therefore virtually impossible to prevent in a flock. All you can do is manage the situation in the best way that you can. Your bird may be showing a relapse that will resolve itself (I really hope that this is the case), or it may be that she continues to deteriorate and show further neurological symptoms. In the end only you can decide how your bird is doing and what course of action is best. Treating and caring for a bird, or euthanising it, the choice is yours, and depends upon many factors. You have already given your girl a good chance - I hope that she can overcome this setback and continue to live a happy life. However if she deteriorates to the point where you can no longer care for her then no-one would criticise you for choosing to end her suffering.
All the best for you and all your flock.
