We have been uploading documents into SharePoint from the file system by dragging and droping them from Explorer, and we could'nt work out why they were being shown as "Checked Out" even though the Version Control settings were set to not use "Check-in/Out".
The reason turns out to be related to mandatory fields being by passed by the Explorer upload method. If you load a document via the filing system and a mandatory field is not completed then Sharepoint leaves the document in a “Checked Out” state. However you cannot “Discard the Checked Out” state because no document has been “Checked In” in the first place !
So you have the situation where a document is in limbo – not checked in or checked out but only visible for the user who loaded the document to complete the mandatory fields.
BUT even if you do complete these fields – it would seem that Sharepoint keeps the document in the limbo state and won’t allow you to do anything with them unless you fully check them in.
AND the problem is that SharePoint doesn’t have a “bulk check-in” function!
Reference: Automatically checkin files after uploading « SharePoint, ASP.NET and more