One of the main reasons why horses roar is because they are not able to breathe properly during exercise and it's often caused by a condition called laryngeal hemiplegia. This condition can be caused by a variety of factors. These factors include:
A horse might have a genetic defect or trauma that damages or breaks down the nerve that controls the vocal cords in the horse’s larynx. This nerve, which is usually called the laryngeal nerve, is responsible for controlling the movement of air through the horse’s windpipe (trachea). The nerve that causes this is called the recurrent laryngeal nerve.
The nerve might be damaged by an infection or inflammation, such as laryngitis. A horse might also have a problem with his breathing due to a respiratory tract infection or lung disease, such as pneumonia.