Check your client settings and see if it is set to automatically add newly downloaded torrent files. I get this sometimes if I accidentally download the .torrent file twice.
What happens is you get 2 copies of the .torrent file in you downloads folder in Windows. When I delete a download from my client it's set to also delete the .torrent file. What can happen is that if there's a duplicate in the watched download folder it will immediately pick that up and start downloading it.