Owners will carefully manage their horses turnout regime in the field, who their horses’ field mates are and they will balance the importance of social interaction with preventing injury amongst their livestock. They will know that causes of aggression might include dominance over food, water or attention and will typically manage this accordingly - such as to space out feed or hay allocations, allowing a safe space between each animal.
This careful management can be disrupted if someone decided to feed the horses over the gate or fence and the horses might collect in a small area which puts each other at risk. Some horses have suffered catastrophic injuries from situations like these, such as kick wounds, or fence / wire cuts from fleeing from an aggressive / dominant field mate.
Flash is an example of a horse that suffered a catastrophic injury and is still recovering 10 months later. You can find more information about how owners can manage their livestock to prevent field injuries here.