Does she do this when she is alone at shows? Sounds like she is just nervous at shows, which is pretty common. Have you tried any calming agents before a show?

>>My 12 y.o. Morgan alpha mare whinnies and is totally non focused on her job when I take her to a show with one of her barn mates. She does not pasture with this other mare and in fact bullies her when she was out with her. they were trailered to the event with each other. thanks, DG