My granddaughters barrel horse started doing the same thing when cinching her up. Acting like she wanted to go down, spreading her legs, turning around to bite, and believe me, she got us all a couple of times she was so quick. No matter how we cinched her up, even tried a couple of different types, but nothing worked. Charmayne James was having a barrel clinic and had her personal chiropractor there. We had him work on Ruby and found out her neck was out just at the withers, after he adjusted her, she never gave us another problem. He said it can happen easily when they run and play, slip when running, rolling, etc, just a thing that can put it out of place. Maybe he has the same issue.