Related question: what's the deal with having a login form that only takes a username, and then only shows a password field after you've pressed enter? I find this very annoying and can't come up with a benefit.
SSO handoff as a sibling points out but also once email is known, a determination is made about trust level for the sign in attempt. Cookies, IP and other fingerprinting are used to determine whether single or double factor auth is required.
If username is an e-mail then an app probably checks if e-mail domain has configured single sign on (SSO) and, if it has, it redirect you to your company authentication server instead of asking for password.
