I think after this post I'm going to change my password to something a lot stronger (though I couldn't tell you off the top of my head what my current one is).
I've seen people who use a few different passwords, but don't remember which one they use where. So they just try all of them and see what works.
If that is what you're doing, you're basically sharing all your passwords with different websites, which kinda defeats the purpose of using more than one password.
If you're using a password manager and look it up, or have your browser store it, this doesn't apply to you.
Have you recently been redirected to a Bitcointalk login screen (or at least what looks like it) via a link in a Bitcointalk PM you've received? If so, someone might've phished you out of your Bitcointalk password.
One very simple way to prevent this, is having your browser store your password. If I need to enter my password instead of just clicking the Login button, I know it's not the real site.