Why Is My Horse Rubbing Her Tail?

I have a 16 year old standard bred mare who I have owned for eight years now. She just recently started to rub her tail out. I have wormed her multiple times but she doesn't look 'wormy'. She is eating normal, and acting like nothing is wrong with her at all. I have tried washing and conditioning her tail thinking maybe that it was dry skin. She is kept in the barn during the daytime and turned out to pasture in the evening/all night. Do you think it could just be boredom? What can I do to stop this?