This is related to how browsers autofill information when they think they've found a username or password field. It’s a security measure that browsers take to prevent people as much as possible from typing passwords. Essentially, the editor is a "trusted website on a trusted domain" (bubble.io) so it inherits all the saved passwords/emails and possibly other form information that you have. Since browsers have saved this information, they then try to autofill it for you.

For example, Chrome has some secret logic to detect a password on a page (any input type = password). If there is one, they’ll take the first visible text input and assume that’s an autofillable email. That's why one of the main places you may have seen this is when trying to add a new field to your User data type.