I don’t know when. But happens that if you are one of the first comments and few several people set like to your comment or reply to it… that comment has good chances to continue first for a long time.
This makes that behavior, a SEO strategy. You can comment first: I like this video because it is very inspiring for my channel blah blah blah
If your comment remains first and many people see it and the video is seen by many people, then several of those people will go to your channel.
Then most people try to set the first comment. As it is difficult to be the first in channels with more than 1M subscribers, then several people also compete for nothing to be the first.