I don't think there's anything special about the field -- I would guess that maybe 1Password has an algorithm like "if focus moves from a password field to anything which is not a one-line text field, prompt to save the password". That would let users use normal forms (which have a one-line text field called "username" or "email") but do the wrong thing in this case.

If 1Password respects "autocomplete=off" (or has some similar hint), we could conceivably add that to the password field.

This post on StackOverflow suggests it does not, though:

I use password/form managers like 1Password and RoboForm specifically to get around websites that disable autocomplete; these add-ons typically ignore the website's preferences in favor of their own more sophisticated logic.