It seems someone obtained your private key. If you are sure you did not authorize this transaction, consider that paper wallet compromised and never use it again. If you generated the wallet online (just by opening the website), your keys may have been stolen by malware / keylogger of some kind.

If you need to generate a paper wallet, a better way is to download the source code and run it on a computer disconnected form the Internet. Disable all browser extensions or use a freshly installed and up-to-date browser (e.g., if you usually use Chrome, install Firefox just for this task). Check your computer for viruses.