Basically, The feature that when you flag a post, it also posts a comment to said post detailing why you flagged it, has recently been added to the SE software.
This caught me a bit by surprise, as it changes the existing behaviour of the SE site, and I wound up having people respond to comments that I wasn't aware I posted.
I think it would be nice if there was some information somewhere in the flagging dialog that flagging the post will auto-add this comment.
For that matter, can we get an explanation of what specific actions trigger this macro in the first place? Is it for every possible post flag, or only some of them?