If you are an authorized user, the account will be listed on your report even if the account is only in her name. You can contact the lender and ask to be removed as an authorized user. Or, it could be a joint account and you forgot that you applied for it to be joint.
If you download your full credit reports at www.annualcreditreport.com, they may show in more detail if you are a co-borrower or authorized user.
If your wife has bad credit and you have good credit, adding her as unauthorized user to your cards may be a way of raising her credit score.