It could be because of many different things.
2) May be there was another task using the same files
3) Antivirus scanning those very files (make exceptions in your AV program).
4) some inconsistencies in that directory.
It is fine to stop the QDS service, remove/backup the content of C:\ProgramData\QlikTech\DistributionService\ and start QDS again. The folder content will be automatically recreated and any temporary files would be cleared. The directory holds temp files, reference files for the repository and log-files. No task settings would be lost, only task history.
Let me know if that worked.