I have 2 thoughts - I do equine therapy for a living and it is common for horses to be "touchier" in the winter due to their muscles being colder. They tend to be a little more tense which will cause any restrictions or soreness they have to be worse in the winter months.
Try checking her over in more areas than just her back.
I have a section on my website that tells you how to do a thorough evaluation of a horse for restrictions, compensation patterns, etc. Learn How to Perform a Thorough Equine Evaluation
You could try doing some of these tests and see if you can find anything.
My other thought is - my younger horse started doing some of this and we think he might have an ulcer. I tried moving him to a new farm and he flipped out, so we had to move him back "home". Ever since he has been acting "funny", the vet thinks he might have one, and the treatment for ulcers seems to help him. I have not had him scoped though.
Anyway, I guess the tightening of the girth bothers them because it pushes on their stomach. Just a thought. I am not currently riding him, so I don't have much more to say about this.