Ask HN: How are posts being reparented?

I've noticed twice now, and commented once about it here with a picture of the post hierarchy before the reparenting happened:

https://news.ycombinator.com/item?id=20246152

Here is the picture of the hierarchy before:

https://i.postimg.cc/W1ytffL2/hn-post-moving.png

And here is how it is now:

https://news.ycombinator.com/item?id=20244726

Is it automatic, or done by mods, or is there an option to do that that I've missed or that is gained on reaching some karma threshold?