Why did my password change automatically on my iPhone?

I'm sitting on my computer and look over and see my phones on the lock screen. It had a few random numbers typed in but was still a 4 digit code as I set it, but when I go in to put in my passcode it says it's incorrect. I understand forgetting your password, but my password was my daughters birthday? It automatically changed the password on my apple watch to the one I constantly use on my iphone (my daughtersbirthday) when I reconnected them a few months ago. I think it's ridiculous to have to reset everything on your iphone just to reset your password. It should simply be possible to sign into your apple account online and reset it from there. And of course the only other ways to reset it are to have another apple device or go in person to the store itself.

[Re-Titled by Moderator]