It could be slight tightness in his back and isn't fully relaxed and swinging. I agree with Hannah to get a Chiro or some other bodywork. Your horse could also be a bit excited or I know when I do extra long trot poles with my horses, their tails come out a bit as if they are lengthening their whole body. It all depends on where the tail is,if it's straight up, or more out behind.
Hes a quorter horse and a chiropractor could be a could idea. Thank you:)
Could be a number of things.. what’s your horses breed? Chiropractor could hold some answers.. but it’s probably just a habit or a personality thing🤷🏼♀️