Hi Mr. Jan,
As I understand, you were uploading multiple documents to a folder using the Explorer view. It works fine, however, the documents are checked out after uploading. Is that right?
With the Open with Explorer feature, we can manipulate the files in the folder in Explorer, it’s ok to upload multiple files to the folder using Open with Explorer feature. The following reason can be caused the documents be checked out after uploading:
a. If you are moving or copying files to a library that requires certain file properties to be filled out, and those properties are missing, the files remain checked out to you. When you fill out the required properties, you can check in the files.
b. If you are moving or copying files to a library that requires files to be checked out, the files are initially checked out to you. You need to check the files in before other people can edit them.
To have the documents be checked in after downloading, if the above situation is meet, please change them.
For more information, you can refer to the following article:
Copy or move library files by using Open with Explorer
http://office.microsoft.com/en-us/sharepoint-foundation-help/copy-or-move-library-files-by-using-open-with-explorer-HA101811182.aspx#_Toc268178633
Thanks,
Evan Zhang