New Sign-in window is opening when I access Amazon in Safari.

I don't know what means. It didn't used to happen. If I click the X in the upper corner, the window closes and I can sign-in normally. It appears now every time I access Amazon via Safari. It's the only website that does this. I would prefer that I just keep signing in using Keychain like before. Does anyone have a better explanation of what it is trying to make me do?