oh my goodness, that's just adorable!
Several reasons
1. Could be just a weirdo lol
2. Boredom
3. in addtion to number 1 and 2, she could be trying to get to her tail to clean it. (Chacho does this when he's in an exceptionally crazy mood and wants to clean his misbehaving tail, though we don't have stairs in our apartment so he just spins around in the tunnel of his cat tree.)
4. a more serious posiblility could be that there's something neurologically wrong with the cat, or some other health problem. In addition, her being bored and having something else wrong could both be a reason. If a cat's mind isn't kept busy, it can start behaving oddly and go into depression or some other abnormal mental state and can do things like that. Hopefully its because of the other possibilities and not this one though.