I worked at a barn and there was briefly a big chunky quarter horse there who was a really sweet, sensitive guy but had been really "cowboy'd" on, so he was always a bit scared of people at first. When you would tighten his girth, he'd sort of fall over backwards. He wasn't rearing up or pulling back, just sort of sitting down, like his hind end would stop working. The first time he did it I almost freaked out, and it just became a process of very slowly and gradually tightening the girth. Each time, he'd "sit down," and I think he eventually went to a home as a pasture pet, but it was always interesting.
You may want to ask a vet about it, just to be safe, but I think gradually tightening it and just taking your time can do wonders. Walking around a bit can help, too, in between tightenings.
The hills were bathed in moonlight, the shadows not so stark;
Silver light reflected off his brown hide as he held me in the dark
– I love you, Mudpie!