The second reason horses rear is rebellion, evasion, fear or one of a number of mental problems. A horse who rears for one of these reasons simply has not been broke. It is not willing to accept the handler's direction. It may be rideable, but it is not broke. Even the most experienced of us can be injured doing this, not to mention the safety of the horse.
Mechanical corrections are nothing but band aids.
They may hide the problem for a bit but eventually the band aid will fall off and the problem will be worse. After an hour or so of bonding, the horse can then be examined thoroughly for possible physical causes that may be more discernable once it has a willing relaxed attitude. I have a bonding scenario that I use. It is also the mainstay of my Mind Meeting Mind Horsemanship method and what I teach my clients and at my clinics. If you would like a copy, please e-mail me and request the bonding scenario.