I believe the notification did not get sent out for the first PR until a comment was posted on it (the PR was opened 2 days ago).
I think the top commenter on this thread (Rob) may not realize his comment is the one that is triggering the first wave of notifications, based on his choice of sound effect.
NSFW warning: Someone has commented on this PR with goatse. Do not click at work.
I just opened the email, didn't need to see that. The image embedded and rendered in Gmail.