It could be many things. I'll fitted tack that pinches the withers or scapulae (even though this usually shows up once the horse is moving).
Most likely it comes from when the horse was started and girthed for the first time. The trauma stays for a long time.
Put a rope around her belly, where the girth would be and tighten, if she reacts it's the girth and not the saddle.
could be ill fitted tack causing him pain. then again, could just be a grumpy horse.
The horse could have ulcers or a bad experience with a girth!
Has he seen an osteopath? That's maybe because of an injury he has on his back? If he didn't do that before, there's a problem