This is a browser feature to prevent your login credentials potentially being stolen by keyloggers. When you've logged in, it saves your credentials so that you don't have to re-type them again whenever you return at a later date.
I use Chrome, but you can find the same shortcuts to whatever browser you use too. Here are some options:
- Press Ctrl+Shift+N to open Incognito mode and try this again - it should ask for credentials
- Open your browsers settings page and tell it to forget all password / login form data
- Open another browser (like Firefox or Edge) and test this with cookies / cache disabled
Upon logging in again, the browser may ask if you want to "Save" those credentials for that website. Simply select "Never" and it won't ask you again. You will always be prompted for your username / password combination