Your diagram will be laid out base on its characteristic. Different types of diagram will have its own layout heuristics. Furthermore, even the same type of diagram with different amount of models will have different layout effect when applying automatic diagram layout.
Complicated class or collaboration diagrams can be rearranged by applying Automatic diagram Layout. Visual Paradigm will re-design the layout for your diagrams. For class diagrams, Visual Paradigm will analyse your class relations and show the class hierarchy in a new layout.