I'm guessing it's a function of the browser the poster uses.

The name of the web form field is fairly generic, which means if the poster filled in a value in a field with that name on some other Web site, their browser auto fills in that field on this site when they make a new post. The password field is not initially visible on the "new post" page, so the poster won't know that it was automatically filled by their browser.

This is just conjecture on my part.

Cheers
Eddie