Check for back pain and saddle fit. My guess is the saddles doesn't fit well and as a result the horse is resistant to being saddles. If the horse continues to be ridden in an ill fitting saddle, eventually the horse will like start to buck and act out due to back soreness. An ill fitting saddle can cause all kinds of issues, that people commonly assume are "bad behavior" when in reality it's poor saddle fit and the horse is trying to indicate that they are in pain.