.nfsXXXX files appearing, what are those?
Nothing is going wrong. This is your NFS client trying to maintain proper "delete on later close" unix behavior within its own operational abilities. This NFS behavior is known as "silly rename":
http://nfs.sourceforge.net/#faq_d2
NFSv4.1 will get away from this behavior with OPEN4_RESULT_PRESERVE_UNLINKED:
https://www.rfc-editor.org/rfc/rfc5661#section-18.16.
Adamo is correct. Make sure that your app is closing the files once it is done with them.
If you can't fix the app for whatever reason, you could create a cron job to regularly remove the files.