Drop-down menus should use common fate with a consistent flow of movement. 

University of Michigan - Ann Arbor

Common fate can be used for designing drop-down menus or illustrating a process.  You should make sure that elements that belong together move in the same direction with the same speed.

Common Fate in Interaction

"What are Gestalt Principles?" Interaction Design. https://www.interaction-design.org/literature/topics/gestalt-principles

Gestalt Principles of Design

Example 1

This image shows both the way to use common fate in drop-down menus and process illustration. 

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