I am frequently confronted with behavioral issues that are directly related to equipment. Ear pinning, nipping/biting, kicking, bucking, bolting, snatching reins, reluctance to move, move forward, rearing, loss of athleticism or quality of movement are all potential issues which can be caused by dislike of equipment, pinching, ill fitting, too big/wide, too small/narrow. Some horse are more opinionated/sensitive than others. TB's and Arab or blood type breeds can be more disturbed by poor equipment. Other types may tolerate it but perform poorly pinning ears and not wanting to go forward.
Most commonly, I see poor saddle fit and sore backs. It is seldom even thought of, in some cases that a saddle would even need to be fitted to a horse. You just go and buy a saddle you like and put it on and ride. Just imagine wearing shoes that caused blisters and made you feel crippled but you had to wear them day after day and worse run and jump in them!
I'd like to highlight an example of my own horse Mercury. He has a problem with if the girth does not have enough stretch/elastic in it. I found that he doesn't breath well and then doesn't move well and then has the urge to buck.
How did I figure this out? Well I like watch horses go with and without tack a lot. I free lunge regularly. I will sometimes have them saddled while they are lunging and will look for marked differences in there way of going. In his case, he bucks and kicks at the girth initially and I noticed a drop in his quality of movement versus when he was un-tacked and free. Also, I knew early on, that he didn't care for the saddle much as he accepted me bareback more easily in the backing process. When I saddled him for the first time, he bucked with a ferocity I had not seen in any other young horse I had started. This continued for an above average length of time and still does initially hence his nickname "Rodeo". I have learned to listen to his likes and dislikes (as he's quite dramatic sometimes), so now we have an understanding and have moved through these obstacles. Always remember they have no other way of telling us they are bothered by something. So always start your bad behavior evaluation with a physical exam followed by and equipment check. If you don't know how to do these things, consult an professional.