A reCAPTCHA is displayed when you did not choose to display it because Marketing Cloud Account Engagement pings a database of known spammers and when a match is found automatically displays a reCAPTCHA. This is a security measure that helps prevent spam bots from submitting your forms and creating fake prospects. You can also choose to always display a reCAPTCHA on your forms or form handlers, or to never display a reCAPTCHA on your forms.