Turbotax is part of Intuit, and Intuit uses a single login for all their sites. So if you had an account with Quickbooks, or one of the other help bulletin boards, you automatically have an account here. Intuit also merged with Credit Karma, so if you have an account there, it may have been merged to Turbotax.
If that doesn't explain it for you, you can contact support and ask to speak to account security.
https://support.turbotax.intuit.com/contact/
