It looks like I'm being notified of comments to questions/answers I commented on, without the replies actually containing "@Matt" or similar.
I couldn't find any recent Q&As on this behavior. Is it new? (I like it.)
It was implemented as per here. If you are the only other commenter on a post, comments from the post author will trigger a notification to you even if they forget to use the @ syntax.
I actually got two notifications this morning. It's a very nice feature.