Hair loss is normal to some extent, however if you are worried that she is loosing more than normal you should check her skin for signs that she is scratching it out and consider who whole diet for nutritional content.
Chronic selenium toxicity (levels slightly over what is required) can lead to loss of mane and tail hair. External parasites can lead to itching and pulling out hair, skin infections can lead to hair loss, etc. So you need to take a closer look at your horse and your horse's care.
Licensed Veterinary Technician