-She probably experienced her first orgasm while riding, (clit rubbing on the saddle) so she has a permanent emotional bond with the horse.
-Her hymen was ripped off during riding, again tying her to the horse forever.
-They start at a young age, before the brain and skull is fully formed, and all the bouncing rattles the brain with minor concussions repeatedly throughout adolescence and into adulthood. This prevents the brain from fully maturing, primarily the pre-frontal cortex.
http://en.wikipedia.org/wiki/equine_induced_dementia