Most likely :
1. You create your paper wallet with malicious software/service
2. You create your paper wallet with online website on insecure device/connection
3. Someone found your paper wallet & decide to stole it
But those are educated guess, do you mind tell us how do you create your paper wallet & how do you store it?
1)probably
2)probably
3)absolutely not
well i created it offline, and stored it on a <>