Well I am probably going to get skewered LOL but in my many years around horses I find lots of them get cranky when the girth is tightened up. I know of several who fall over and play dead if you tighten the girth too fast, that is the extreme. However signs of disagreement during saddling are common. Very often horses do it when they are not sore and it does need to be addressed.
I mean am I the only one who knows of many horses that aren't really fond of that part of tacking up? Once you get through that they are fine and go about their business. Yes it needs to be addressed you don't want to get bit and let the horse get away with it. (unless of course its a pain issue)
Just my two cents for today.
I got a horse that already had this bad habit long before I got her. She tried several times to bite me when I tightened girth. (and I checked for pain, saddle fit, etc) and determined it was just her protesting going for a ride. After using my voice, etc. I just smacked her the neck very quickly and sharply and guess what she never did it to me again.