My dads horse was my friends champion barrel racer for years, and I know that she rode him in a gag hackamore (don't ask me why), she always had light hands, and more often than not, he was sleeping at the gate before his run. Unlike a lot of other horse's I'd see where the rider had to force them to stare at the wall so they didn't start towards the barrels before the rider was ready, he would walk in, stand calmly and quietly until told to go, but when he went, he burned rubber. He loved to run barrels, but he wasn't stupid about it. He also spend a lot of hours just out on the trails behind my friends house when she had him, and as she got older, and started giving lesson, he and her old mare were her two main lesson horses. She got a few "hot" horses, but she would spend a lot of time working on the trails to get them to calm down and relax. I think it helped that they had another outlet, and they weren't always seeing barrels, and running barrels, and nothing else. I think some of it is training, some of it is genetics, some are just bred to be "hotter" than others, and that "hotness" is cultivated by some riders, and some of it I think is that younger people think its "cool" that they've got the "crazy" horse, that gets all hyped up, and hard to control before a run, and they intentionally ask the horse for that, so that they look cooler than the people who's horses are asleep at the gate.