It can happen in a few ways. You could be reading a Q/quip of theirs and think “HEY! shilolo just reached 10K!” or you could be on their profile stalking, writing them a message or adding them to your ask-public and notice the lurve. Then you search ask-public to see if there has been a question asked about them yet, and if not, you go ahead and do the honors. Also, people who are closing 10K are typically very excited and looking forward to it and thus mention it in PMs to their ask-public friends, in the chat room or even on a thread. This automatically makes other people look out for their lurve score more than normal to witness their achievement. Just from general ask-publicing, I am aware offhand that eponymoushipstr and richardhenry are approaching 10k. I am sure there are others too.