These horses just learn that they are constantly be bumbled around by clumsy people. Everything is rough and out of timing...confusing and inconsistent. I'm sure I would go sour too. That's why I always believe that even the best lesson horses should still have a very knowledgable rider get on every now and then to show them that yes they are doing the right thing, and to keep up on responsiveness.
When I was a freshman my school's equestrian team deals with everyone from people that have never touched a horse to those that are close to professionals. They had one horse that was very very bitter. I was told he charges people in his pen, he bites, he kicks...ect. I handled him a few times and while he never did charge me he threatened to kick several times and bite. I gave him the correction necessary but the poor guy was miserable being a team horse. I just heard that they sold him because he was becoming unmanageable under saddle as well. Something about they kept upping his bit because they were putting newbies on him with strong bits and were making him very hard mouthed.
I'm glad he got sold. He needed a come to jesus meeting then one very consistent person to handle him.