Hate rearers, sorry! It's far more dangerous than bucking, rearers are evading pressure by not going forward, at least with bucking you can give them a good whack and make them go forward. Rearers can flip over backwards, land on you etc. If you come off you're pretty much always going to come off backwards and they can give you a good boot in the head on your way down, where as buckers tend to throw you clear.
As for why they rear, generally rearers are the ones that won't get off your leg. They're evading going forward by going up. I absolutely do not stand for a horse going up to evade pressure, particuarly if it's a regular occurance.
I'd be putting spurs on and carrying a whip, and the second she stops get right up her. Might even help to have someone behind her with a lunge whip to help get her going forward. Rearing is not on, neither is bucking, but again, I'd prefer a buck to a rear.
I have a horse that rears.. he only does it in the spring though. He is also not that young.. 11 acutaly.. he has some of the best training of any horse I know, and he is very gentle on the ground.
But he rears.. in the spring ... He dosn't just do the little hop to turn around, he does the full on rear, strait up, pawing the air. I agree, it is very dangerous.. but he walks, and right after he takes one step he rears. I have never had to bail, for he always goes down, and then keeps walking.. Or he does it again. He has never come close to flipping no matter how high he gets..
So there for it is not a huge pet peeve for me. He is a great horse. And if you know how to ride, then you know how to control him.
I am the opposite to you. I prefer a rear to a buck. I fractured my neak being bucked off a 16.2 arabian quater horse... I did not land far from him. I landed in the path of his feet. He jumped me and kept running.