Is there an explanation to my horse's behavior?

Have you had her since birth or recently brought her into the family? If she has been raised at another home she may have been 'bullied' by other horses or neglected by her owners but if you have had her since birth the only other thing it could be is if you bond with this horse everyday she may be protective of you. Horses are just like humans. Also you may want to take her to a chiropractor because if her back is out she could be playing up. When my horses back was out i had no clue, but when i took him to the chiropractor he came back a different horse. So this may be something to look into. :-)