Some varieties of worms (pinworms, I think) can cause horses to rub their tails. I had one that apparently just had dandruff, lol. I massaged baby oil into his tailbone daily and that seemed to control the rubbing. I have one now that acts insanely itchy all over after a good workout (untacking becomes a speed event before my leather stuff is scraped over everything in reach, lol), face, shoulders, chest, and butt, lol.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown