Does Free Download Manager automatically upload my torrents when I download them? If so, is there a way to prevent it?
About torrents in general, I recommend you to read the Wikipedia article about the protocol.
The bittorrent protocol is a peer 2 peer architecture. There is no main server uploading, only users, sharing with their home connection. As such, the whole thing works because lot of little connections are sending you the file you want.
However, as you can understand, this works only if people are uploading and not only taking. This is why most of clients will upload to you only if you are uploading as well. Many ones will simply refuse to send to you, because you don't "play fair". This is why your torrent files are uploading when you are downloading. To play fair on the network, and as well to increase your chances of people sending to you.
If you want to turn off upload in Free Download Manager, you have to open your parameters (Tools > Parameters), and set limitations for upload, in the Bittorrent category. You can limit the upload speed to 0kb/s, and to 0 simultaneous connections. That should prevent any upload.
However, I would recommend you to leave at least a bit of bandwidth for upload, for the reasons given above, or you will only get a very slow download speed.