Why are all of my SharePoint documents showing as checked out after migration?
Some versions of SharePoint have a default setting that automatically marks documents as "checked out" when for editing. This is important because the process of migration acts as "editing" the contents of the library. It is possible to configure a library that requires that documents are "checked out" so they can be manually edited. In both of these scenarios, the documents that are part of the migration will show up in the destination library as "checked out".This setting can be changed by turning off the enforced check-in/check-out option before migrating your documents.
Note: The enforced check-out of documents causes the last modified date of the documents to be overwritten by the migration. To keep the proper information on the document, we recommend that you remove the previously migrated files from the destination, change the settings or create a new library, and perform the migration again.
Alternatively, if the last modified date is not a concern for your documents, you can also perform a bulk check-in of the files, using the instructions provided here: "Check out or check in files in a document library".