Question:

When does the system request automatically for a re-rendition of a Document? 

Answer:

If a file does not have a viewable rendition and a user with View Content Permission tries to view the file, Vault automatically re-renders the file for the user. Usually, this happens after migrations. During a migration, documents are in a Suppressed Rendition State.

Related Documentation:

Vault Help Documentation: About Auto-Generated Viewable Renditions