How do you stop a horse from doing flying lead on the long side of the ring? He did not start doing it until my trainer had me move off the rail about 3 feet.

My horse does the same thing and I have yet to figure out why. But maybe try doing/thinking a shoulder in along the long side?
I will be bending him with only inside rein and outside leg and going slow but he still does it. I have also tried leaving him alone (going into 2-point).
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