Eh, it's no different than people who are passionate about anything. Car people act the same way. People who get into a hobby that has expensive things you get attached to, you can get protective.
That being said, I was raised on a horse ranch. My mother is a trainer. I've seen countless horse girls. I could count on one hand how many I would consider kind of crazy. Most are some of the most grounded, down to earth people you'll ever meet.
Horses are a lot of work and it takes a lot of dedication. It can build a lot of character. I wouldn't judge an entire group of people because of the few exceptions you hear about.
Also to add, most horse girls have incredible bodies.