Try placing a grand rail out in front and a ground pole out after the fence, nine feet. Work with your trainer for appropriate distance. I can’t remember, I think it’s called the nine exercise for placing the pole nine feet out from the fence.
My horse used to barrel around with me in the jump ring like… well like I was being dragged around. So placing ground poles before and after fences around the course put him back on his feet and enabled a ‘check’ in the riding so he couldn’t just jump and drag my butt around town and helped him to round over the fences. We even at times had this through the gymnastics to help not let him run right through the line.
Check your bit too. He could be trying to pull his head and reins out of your grasp. By lengthening and catching you off guard he may be trying to sneakily take control.