Ask HN: What benefit is there to the enter username then enter password pattern?

Apologies for not knowing the proper term for this, but I have increasingly noticed that many websites are adopting a new login process that I can only describe as "Enter Username, then redirect to Enter Username + Enter Password". In other words, I have to enter my username, then it redirects me to a new page where I enter my password (sometimes the username field is present too, sometimes it is not). This is different from the past where the username and password field were almost always on the same page. I think this might have something to do with oAuth, but I see this behavior on websites that don't use any common account infrastructure. Ultimately, I have two questions for web designers and UI/UX folks. What is the name of this pattern? And, does it bring any benefits or is it just a trendy thing to do?