The practical reason walking under a ladder is considered a misfortune is that it’s just plain dangerous. The more superstitious theory is that the shape of a ladder on a wall forms a triangle, a symbol of the Holy Trinity in Christianity. Some believe that walking under a ladder would “break” the Trinity, a blasphemous act that could attract the devil.
However, that is not the only explanation. Some believe that a ladder on a wall resembles a gallows (associated with death by hanging), and the ancient Egyptians believed that walking under a ladder could cause you to accidentally see a god climbing or climbing it climbs down.