Someone offers you a free Animal Jam account, or you find one on an AJ account website. When you enter the password, your account is breached--usernames and passwords of your other Animal Jam accounts are revealed to a hacker, giving the hacker access to all your items.
Warning: This hack may result in AJ accounts saved on your computer/linked to your email being banned, parent accounts being banned, and passwords being leaked (email, AJ account, parent account, etc).
How to avoid this scam
Do not use free Animal Jam accounts from account sites. It's a good idea not to accept free accounts at all, especially from someone you don't know. If you choose to take a risk, move all your rare items to a spare account that isn't saved on your computer and isn't connected to any email. It is best to change your password as well.