Why is a two-step (not two-factor) login trending right now?

I have noticed that more and more websites and services are presenting two distinct views.

In the first one you input your username and click next. In the second one, in case the username was valid, it finally asks for the password. Then you login.

Questions

• Is this actually a good UX practice?
• Why add two steps if you could make it only one?
• Isn't it disclosing privacy for an obviously existing username in case it changes to the password page?