HubSpot forms pre-populate fields with known values by default. This is done by associating the tracking cookies in the visitor's web browser with their existing HubSpot contact. If you previously submitted the form as a test, you'll see your own information when you visit a page with the form again.
If your form fields pre-populate with another person's information who has never submitted a form on your browser, it is likely due to a forwarded email.
Due to how email tracking works in HubSpot, when a contact forwards an email to a second contact, the second contact becomes associated with the original email recipient's tracking cookie. This will associate the second contact's information with the original contact's browser. This leads to any forms that are set to pre-populate with known values to show the second contact's information on the first contact's browser.
If you or your contacts are noticing another person's information pre-populating your form fields, clear your browser's cookies and re-submit your form to become associated with a new tracking cookie.