If your horse were to take off unexpectedly and you are not able to stop them, circling is a great way of gaining leverage and therefore re-establishing control by turning them into smaller and smaller circles until they stop.
Polo Tip #42: Taking The Danger Out Of “Taking Your Man”
Field Awareness is no different from Driving Defensively. The good drivers are aware of all of the traffic around them and ahead of them. They stay alert to unexpected and sometimes even illegal moves of the other cars around them.