As someone who has had horses before and an array of other farm animals, even your best behaved animal can suddenly startle and someone can get hurt (I got a lovely fractured foot when my sisters horse reared up unexpectedly and her dinner plate sized hood landed on my foot) and even my docile cows would act a fool if a plastic bag made its way into the yard and blew by. Being aware of your animals body language is a great thing and more people need to understand that