What causes a horses tail to break off?

I have been around horses for decades and have yet to see a horses tail "break" off. They can be cut, pulled and scraped off. This is from abuse by humans or other horses, but that is only partial amounts of the tail's hair.

Perhaps you could help us a little and explain what you mean by break off?

If you meant the tail hair itself then that can be caused from several things. The hair could be brittle due to missing nutrients, being dried out by the sun, or even from excessive shampooing of the tail hair. It would be best to consult with your equine veterinarian about what to do for this.