Disorder is more probable than order. Think of it as entropy. Attribute it to the Second Law of Thermodynamics. Whatever mental category you use, disorder is more probable than order and that principle applies to churches as well as physical substances.
Unity in the church is achieved by design. If we don’t think about what we are doing as a body, disorder will be more probable than order. If we are intentional about working together and putting others before ourselves, unity will be the result.