Why do some login flows hide the password field until after you submit your username?

I see this more and more lately: go to log in to some site, and they only show the username field. Enter username, click Submit, then a password field appears. Enter password, click Submit again, and then we're logged in.

This makes using a password manager super annoying, because I have to trigger the autofill twice.

Is there some security-related reason more sites are doing this? Is it an anti-bot thing? I'm just really curious, because it seems so pointless on its face, but it seems to be spreading.