He hollered to make the horse whoa and whipped it to make it stand. It was a huge disaster, due to a foolish error. Careful drivers prevent injuries by matching horse to vehicle, and preparing the horse to handle stresses of the environment. Drivers guard against harness galls by conscientiously fitting collar and harness to the horse. Other safeguards include covering the breastcollar with fleece padding and sliding rubber tips over the ends of a cart's shafts.
Accidents can occur to horses in harness.
It's amazing, when you look at how little room they have going through those hazards, with the width of the vehicle. In any driving situation, the horse might catch his bridle on an object and pull it off, so the driver loses all control. The horse also could get a leg over a shaft and panic before a helper can release him from the predicament. Working in winter, the horse can get snow balled up in his feet and lose his footing. Drivers try to avoid such crises, but adding multiple horses increases problems.