Which actions modify
/posts/row/@LastActivityDate? Creation and edits seem obvious, but what if, say, the post is a question and the owner accepts an answer?
For all posts, the LastActivityDate is set/reset when a post is created/edited. For questions, the addition/edit of an answer also resets the LastActivityDate. Basically, anything that will cause the question to be bumped onto the front page of the site. Accepting an answer does not bump the question to the front page, and therefore does not reset the LastActivityDate.