He has settled into a steady, relaxed rhythm. Maybe you have come off the rail to pass someone, and the ground is softer off the rail. If the ground conditions change, the horse will alter its balance and rhythm. That could mean the class. Still worse, the horse could interfere and throw himself off balance even further.
Many horses carry themselves crooked.
The off hind foot tends to be low on the outside quarter and may toe out more than the near hind foot. Thus, the rhythm of the horse's gait and his balance are affected. This condition is often associated with paddling or winging. The side support shoe can help the horse overcome these irregularities. Also of note, the side support shoe can be useful when the horse has a contracted heel.