There are multiple reasons why a horse might run away from its owner, including the desire to remain outdoors with other horses, playfulness or being disturbed by something in the paddock. Horses do not like to be confined any more than humans do. If given the choice, they want to remain out in the pasture. If a formerly well-behaved horse begins to suddenly run away, then something has changed.
If a horse breaks away and runs off when it is time to return to the paddock, it may be because there are other horses nearby. If a horse sees, smells or otherwise senses other horses nearby, it often resists being confined, similar to a child who does not want to go into the house while his friends are still outside. Another possibility behind the running away is that the horse thinks of it as a game. Rather than being locked up for the night, the horse would rather stay out in the field and play. It’s possible that the horse just wants to be chased. This is especially true in the spring when the nice weather returns.
Yet another possibility is that something may be bothering the animal in the paddock. If something has changed in the paddock, the horse may be disturbed or unhappy when it returns there. Animals are very sensitive to every change in their surroundings.